Admittedly, Toady did (at one point) say that he wanted DF to be a sort of 'cheap fantasy story simulator.' (his words were similar to that - I didn't insert the 'cheap,' before someone wants to flame me to death).
I, personally, would like adventure mode to be a more proactive (on the player's part), sandboxy sort of situation. Leading a band of bandits the way Capntastic talked about would be awesome. But that's pretty much personal preference. Still, I'd like to see what the game's like before a protagonist spotlight gets placed on the adventurer. It may very well be that people are able to make up pretty cool stories without events being forced to happen near the PC.
The reason (as I understood it, at least) that you don't get sieged at 7 dwarfs is because why would anyone spend all the time and resources needed to muster, feed, and transport an army hundreds of kilometers just to attack a few dwarfs grubbing in the soil for mushrooms? From an in-game perspective, it doesn't make any sense. It isn't just for game balance. That's why there has to be a certain amount of resources before you get ambushed/robbed/sieged. If, in the future when the army arc is implemented, your fortress is on the way from a goblin fortress to the city they want to sack, it would maybe make sense if they decided to do a little pillaging on the way. And quite frankly, I think that'd be awesome.