There is a distinct difference between player and NPC fortresses at the moment. Frequently dwarves in other fortresses will stay as lye makers for all of their lives! That almost never happens in player fortresses. Player fortresses thrive from the player input, but even the player cannot expect everything. And now this is going to the "why adventurer":
Because there is a need for Fun.
Players like to play roguelikes, and they like the ordered chaos that is the DF world they just generated. Adventure mode gives you a chance to actually go see that titan (and hopefully slay it), to go see the capital and talk to the king, to drive off ambushes and bandits. It is more "in the world" than dwarf mode, because you can move around the world.
The distinct difference is not 'supposed to be there' but is the result of memory limitations/the undeveloped nature of the game. The reason that fortress dwarves do not stay as lye makers their whole lives is that that the agriculture arc is not done yet, meaning that food is never really an issue; so fertilisers are not needed. As for the rest of your post, yes there is a gameplay reason to have adventurers but that in itself does not harmonise the two modes as one single world.
However, just like any other person, you might be drafted into being a caravan guard or merchant. Depending on what you, the player, want, you might not like this, and could possibly run away. Now wouldn't some other people do the same? They will, so:
Sites will have a higher chance of losing merchants, (expensively)-armored guards, expensively-bought trade goods, and the food that the merchants and guards needed, if they draft people into those jobs.
So naturally if someone just walks up and volunteers to do it? Of course they can be a merchant/guard.
And what about if you don't want to be part of a caravan, just a peddler? Supply and demand have not reached an equilibrium, because of the many dangers associated with bringing a wagon laden with precious goods all over the landscape. So certainly one man/dwarf/elf could go into the wilderness, kill some beasts, and sell their corpses for quite a lot of gold, then use that gold to buy, say, a bunch of silk. Then go over that hardy, desolate mountain range, across deep rivers, and through thick forests, where few wagons can make it, to the human town of Seraelophual, where nobody has ever figured out how to make silk.
See? Trader adventurer, here we come! And we didn't have to neglect dwarf mode players, either!
EDIT: fixed weird quote hijinks
If we can simply send fortress dwarves off to kill beasts and trade there will not be any trade routes left to develop nor will there be any beasts left to kill. Having the system limited by abstact personality factors is going to be confusing and frustrating to new players. If we explicitly tell the new player that this will not work, the effective result is the same as just prohibiting them from doing so outright.
The dangers thing is kind of where I was coming from though. Fortress dwarves refuse to go on long journeys over hostile terrain, this means that while it would be possible to send ordinery fortress dwarves off to trade locally guarded by ordinery militia to trade within a short distance of the fort, for instance to the local hillocks it would not be possible to send ordinery fortress dwarves to go on long trading journeys. This is where the traders/merceneries come in, they are willing to go an indefinate distance away from the fortress and the adventurer trader can simply slip into that kind of role.
You can't use fort layout and traps, you don't have control, you will incur losses. You will not take control of the entire world's trade effortlessly. And with digging invaders your fort will be left defenseless if you try the turtling strategy you mentioned.
Yes you will not be able to take over the whole world's trade effortlessly, because somebody else will have done it before you. We are not talking about the player fortress in particular here, but all sites in the world. Threats from enemies is not going to change anything, because somewhere in the world is a completely safe site that does not face any major threats from any serious adversities, that site then promptly takes over the whole world's trade.
[/quote]
Yes you may not be able to take over the whole world's trade effortlessly, but only because somebody else will have done it before you. We are not talking about the player fortress in particular here, but all sites in the world. Threats from enemies is not going to change anything, because somewhere in the world is a completely safe site that does not face any major threats from any serious adversities, that site then promptly takes over the whole world's trade.
The effect of external threats is solely to move ownership of trade from the less safe to the more safe sites; that situation as a whole remains unchanged.
You keep mentioning meeting "market demand", but the specifics are only a thing that exists in your head at the moment. It's not a flaw in my model, it's an issue in of itself that would affect every profession if such a system were improperly balanced.
It does not matter what the specifics are. If the sites have been independantly able to meet all demands that can be met given the total resources available to the accessable area as a whole then the adventurer trader cannot exist.
Why do you presuppose the sites have the means to saturate the market? Maybe some sites have a skilled labor shortage due to living in a world where people are dying all the time.
I pressupose the existance of surplus value because without it there is no trade at all. If no site produces more of anything than they themselves consume then there is no trade at all for anyone and the topic is hence redundant.
It's like you're asking why somebody would train and equip bandits. It's because that somebody is the bandits/mercenaries. Mercenaries are visitors who can be hired for coin. If they settle permanently at a site, they're not mercenaries anymore.
That is not how the game works, merceneries do settle at a site and they remain merceneries. Unless merceneries do something for the site that ordinery militia dwarves do not do then that does not make any sense. They are more than just migrants for hire, they are distinct group that is quite happy to join up with adventurers that visit their site unlike regular dwarves.
See above. They trained themselves because they didn't have/desire the opportunity to be trained as a fancy militia.
To what end? Nobody particularly cares for them nor their services since they are just fancy militia in social function. When people hire 'merceneries' they are merely thinking of what one more militia dwarf is worth; the answer would be virtually nothing.
You can't assume direct control of ordinary fortress dwarves. If you do, then they're no longer ordinary fortress dwarves, they're adventurers.
Or active militia dwarves.
There is no practical difference, other than that you're trading availability for responsibility (stresses/needs.)
I propose that fortress dwarves be modelled as a seperate group to a group to adventurers but that adventurers that settle/retire at a site be able to do jobs related to the world outside of the fortress borders, which includes manning caravans. Fortress dwarves sometimes decide to become adventurers of a given type in the wider world, normally going off for a time to explore the world and if they survive coming back to become on-site adventurers with can be made us of for their functions. If the player gives them lots of stuff when they set off then they will almost certainly retire back at their home fortress, if the player does not facilitate their choice however they would end up someplace else instead.
That's where you're mistaken. The average adventurer leaves a site (often a hamlet) with very little skill and hones it on the road. I'm pretty sure the hamlets are glad to be rid of them, especially if there's an small chance they'll return home someday being actually worth something.
I do not know what you are talking about; an adventurer is usually a highly skilled warrior and can also be a fortress guard too. While they are off wandering the world on adventurers, enjoying a near 100% chance of coming to a sticky end they are putting their home hamlet at risk and depriving it of his protection. Why would they put up with this state of affairs unless as an adventurer he were willing to do tasks for them that their own core population are unwilling/unable to do.