Depends on the system I suppose.
In 1st and 2nd ed, You cold have a character at 9th level when I start mine at 1st. By the time you hit 10th level, I'll be 9th.
Sounds weird, but that's the way the EXP per level worked. In that situation losing a session's worth of EXP means almost nothing, unless the players are in a race for power (which is another problem altogether.)
3.5's advancement is the same as Pathfinder's "fast" progression, and that's another kettle of fish.
In our 3.* games we handle disparate character levels by scaling Exp: The system awards more EXP to the lower level character for the same challenge, since it was harder for them to handle, so if I give appropriate EXP to each PC based on their individual level, again the level gap tends to self correct. A bit more work, but it does handle the problem.
I've seen games where some conniving players try to play "EXP leech", having their characters show up for a scene, but hang back and do nothing/take no risks when the kettle comes to a boil. They want matching EXP, but no risk and guaranteed PC survival. I like to discourage that, personally.
Over all, I don't award Exp when a player/character skips a session, and I award bobus Exp or penalties for characters who perform above and beyond or below and beneath the call of duty. When they miss a game entirely it means that the encounter EXP is being split amongst fewer PCs, so everyone else's share goes up a bit.
But that's just me