I personally think that if you're at one of those "critical levels" where when you ding the party's experience is literally going to be cut in half, it's ok to leave as soon as you ding, as long as you give plenty of advance notice and look for a replacement. The way I see it, no matter when you leave there's probably gonna be someone in your party who's 1k away from the next level and doesn't want you to go. If you ding and stick around for one other person in the party to ding too, when that person dings there's just gonna be someone else in the party who's suddenly close to the next level, and so on. If you wait for everyone to ding to the same level, you could be there for a long time getting crappy exp.
Many times (at lower levels at least) it's actually more efficient when this happens to just keep fighting with 5 players killing VT's (which would now be T or EM to the player who just leveled) while waiting for a replacement to come along. There may be slightly more downtime etc, but not having high level player anymore means you're getting the same, if not slightly more experience that you were before. Often if I'm party leader and someone else in the party dings to a "critical level", I'll politely tell that player that he's not obligated to stick around if he doesn't want to, unless he wants to build up a small exp buffer before leaving.
Of course, I realize this argument may be a bit one-sided coming from a WHM. When I'm party leader and a melee dings and leaves, it's usually not too hard for me to find a replacement for that person, and we can often keep fighting the same mobs with just 5 of us until a new person arrives. I'm painfully aware though that when I'm the one who has to leave, the party often ends up disbanding. If someone asks me nicely to stick around after I ding so they can get 1k more exp for next level I usually will. However if it's been a mediocre party that's been getting 2k exp/hour and the rate's about to drop to 1k exp/hour due to my new level, I don't feel that I'm obligated to stick around for another full hour if I need to log off and go to sleep.
I've been in numerous parties that have disbanded when I was 300 exp tnl, and I never make a big deal about it or complain. I just figure that next time I log on and join a party I'll probably get 2 levels instead of just 1. I also don't care that much about getting "buffer exp" after leveling - if I die afterwards and delevel I've still lose the same amount of exp that I would have if I had the buffer, and the next time I get in a party I'll just regain my level in the first couple fights.
Anyway, that's my 2 gil on the subject.
First off, calling the Nobel Peace Prize 'political' is like calling water 'wet' - The Nobel Peace Prize is and always was an opportunity for the reformed vikings to prove that they like warmth and fuzziness and have turned their backs on raping sheep in Yorkshire at axe point.