I definitely think that finding people on the net/in forums or being in a good clan/guild is going to produce a better experience with a higher % chance of success concerning the raids, but it seems unnecessary to force people to go outside the game to find 5 people who are able to do a particular raid with you. It's just not what we're use to anymore, and it's strange for the option to not be there. I've found people in games to be useless, but I've found randomers in games to be incredibly helpful and co-operative too, it's luck of the draw. They could even have a rating system to put people off acting like twats in these co-op situations, a DNF % (did not finish) or a thumbs up rating or something.. that way you could see who you were working with from the off and choose whether to invite them to your fire team or not. That probably brings up other issues mind, but it's definitely one way of doing it.
If they manage to have an easy in-game way of finding friends to do this stuff with then fair enough. I'm not sure they will though.. I'm still gonna nail the PvE side of things, but finding a good clan is gonna be necessary for the end game stuff it seems.