Okay, this is more of a question than some brilliant proposition. I think I need more coffee because my brain is muzzy on this topic. What I'm wondering is, since this is a social game that generally expects a group of people to play, why doesn't the group all level at the same time? Why does it matter, if in a party playing D&D, a thief is 2nd level while the magic-user is only first?
Is it because one player might suck and is not pulling their weight and because of poor play is a level behind everyone? That's not really possible in old school play is it? If you're playing and the party gets loot, XP is divided. You would either have to constantly be splitting the party or be absent.
Okay, so you have to play to level up and if Joe Blow only shows up every tenth session I can see why he shouldn't deserve to be the same level as everyone else. But a party with too disparate levels really isn't tenable is it? Eventually if Joe keeps coming he's going to die facing the increasingly difficult challenges the higher level party is facing.
Is it because it is an attempt to manage the different power levels of the classes? (Oh no, the dreaded balance!) Presumably this is because a wizard gets very powerful and will start tilting things into the mid-game very quickly. But couldn't you then just shift an MU's abilities up a level or two? Or not. What would happen if a party of a cleric, MU, fighter, and thief all moved up to second level at the same time? And from second to third? What difference does it make to the game, other than a lot of bookkeeping, that one character might level 2-3 sessions later than another?