There's actually a few common arguments against the LFG system used in say, WoW.
Mainly that it utterly kills any sense of actually communicating and making friends by meeting people. It just turns all dungeon running into another mindless grind with people who want to do nothing more then run through the dungeon as quickly and efficiently as possible. And in a way you ARE forced to use it - because if you try to ask for it in chat, people will just tell you to shut up and go use the LFG instead of 'spamming' them with your requests.
Not every MMO needs to be of the mentality that you need things now. Sure, it does kind of suck for people who play at awkward times or have very little time to play but... you can't please everyone, which is a sad (and sometimes happy, considering some people's idea of being pleased is at the expense of everyone else) fact of MMO games.
What sense of communicating and making friends by meeting people? I've run lots of dungeons (everyone who suggested AC story was just bad was right, the rest seem fun) - sometimes people are chatty and joke around, sometimes it's all business no talk rush through. In 0 cases have I actually ever grouped or talked with (or even, as far as I can remember, even seen) any of them ever again. Dungeon finder would make the situation worse how?
As for people telling you to shut up and use dungeon finder - well, that's because spamming chats for groups is actually kind of annoying and stupid.. but there's no alternative right now so people just put up with it. Dungeon finder is the same thing, but automated and better in every way, so why do you oppose it yet say spamming channels is good?
Meanwhile, there's still guilds and friends lists if you want to form a circle of friends to run dungeons with (I'm about 75% sure you can group up with people on different servers and do dungeons right now, and if not you will be able to for sure whenever they get server guesting working).
Well, current dungeons in GW2 really have to be dumbed down to a lower common denominator.... and have a very large active population of +exotic geared players(we are assuming they earned that gear and know how to play/know the dungeon) actively taking part in the Dungeon LFG...
You will get many many horror stories of fail PUGs and Assholes in the Dungeon LFG... and encounter many of em yourself within the first 6-12 months of its inception. After that, well, it should only be assholes if +80% of everyone else still running is +exotic geared...
Yea, you are opening up the dungeon to a larger player base who normally would not do the dungeons... there is no entry barrier. (Socializing and some effort on the player's part.) They tend to fail harder on a regular basis.
There is also the fact that players get less rewards for doing the same path within a dungeon more then once a day... and if the PUG group wants to go one way... and a player has already done it, they are screwed. (Likely to be quitter debuffs.)
Dungeons are already "dumbed down" - you can respawn zerg every encounter in every dungeon I've seen so far (full disclosure: I'm not level 80 yet). Will you get fail PUGs and assholes? Yes, but you get those right now with the chat spam method - how will dungeon finder change that?
You say entry barrier like it's a good thing. Why? Why do you think some people, who payed just as much as you did for the game, should not be allowed to play all the content? Why do you think they should be locked out? Who cares if they fail - they will either get better and eventually win, or they will give up and the situation is exactly the same as it is now. Plus, I'm not sure "plays during the night" or "picked a low pop server" or "wants to do other content while LFG instead of spamming in town" is a good choice for an entry barrier.
The problem with paths and rewards is a problem right now too. It would not be a difficult one to solve though, if it included checkboxes or something then dungeon finder might actually make the situation significantly better, not worse.
Not seeing any compelling arguments here, or even a single advantage to keeping chat spam as the only group finding mechanism