If I were running 3/3.5/4E, with it's crazy complicated encounter level production balance system, I would probably go XPless. But because the game I am currently playing is 2E, I use XP, and I like it too.
About 50% of the xp I award to my players is for keeping the game moving, roleplaying well, innovative ideas and including other players. I publish a little tally online every week so they can see how much xp they got for defeating this monster, solving that riddle, interacting with this NPC, coming up with that cool idea, etc. I think it makes my players feel that the effort they put in to roleplaying is appreciated in some tangible way.
Of course, that means I have to measure their RP, which makes the xp system pretty arbitrary at the end of the day. I try and be fair and make the better RPers work harder for their xp, and the newer players or shy players get more for less, so no-one really falls back too far behind.
Also, since characters in 3/3.5/4 all level at the same XP amount, it makes more sense to me to level them at the same time. Whereas in 2E, different classes level at different XP amounts, so it makes less sense to level everyone at the same time.