Actually I have heard about vanguard recently... but I digress.
I agree with you 100% that you shouldn't be forced to group up to level. I don't want to group up to level all the time either; sometimes I DO want to go do things solo (when I don't have much time, or could be interrupted, or whatever).
However, in this game leveling is heavily skewed towards questing (till you run out of quests but that's another issue). Dungeons are kind of meh for exp. With the queue times you can't really say "I'm going to grind dungeons now for exp" unless you're a tank.
As for FATEs... they're such a terrible experience that why don't we leave them out of the discussion altogether.