A lot of times, this is what we do. We might be playing 2-3 characters who know each other, and decide which one's we're playing for a particular adventure. The others might or might not make an appearance, depending; a handfull of them are sort of held in common by the group, sort of like super-NPCs.
When someone can't make the game, someone else plays their character; they leave their sheet there for that reason. There's no reason to penalize someone because it's their anniversary or something.
It's certainly possible to go that further step and play as a group and have all characters held in common by every player; Ars Magica does exactly that.
Having reserve characters stay at the same level in case of a new player, or sit in player is different. Expecting to be a part of a game and rarely show up and get to play the same character is another. You should take whatever character the DM hands you this week, if you are not going to be a part of the game continuously to actually be a part of the group and just show up hit and miss expecting to get all the same benefits of the rest of the party.
The problem with all gaining a level at the same time can be shown best in the old example:
The party thief is out doing something to gain some extra money while the other 4 party members are drinking it up at the bar. The thief pulls off his bit of roguery and in the bar you hear <ding> <ding> <ding> <ding> as everyone gains a level. Everyone doesn't grow at the same speed and giving out XP at the exact same rate for everyone is silly. Let alone those not caring enough about the game to show up when/if they feel like it and expect to have the full benefits, it just doesn't work in the game that everyone learns the same things at the same speeds.
It doesn't mean that there should be someone several levels behind, as this would cause DM problems with encounters, but the freeloader should probably be booted form the group rather than acting like and XP sponge.
I have seen a few people think this would happen in the past with older XP award systems and they got mad thinking their character just got free levels, when instead their character was just sitting in the bar drinking.
It also really depends on IF you want someone else playing you character. Many people find their character to be a personal thing, rather than a playing piece which it becomes when everything just rides the level wave in with constant changes in XP for everyone to be equal.
The DM should always have reserve PCs for drop-in players, but that doesn't men that a drop-in should expect to get the same character every time. That is one of the penalties for not being their all the time.
What do you do if only one player shows up and the DM? Just let that player play all the characters and everyone else get XP for just not showing up?
It is a part of the game that XP reward for participation, not just something you get to play with. If that was the case then why have it at all? There will always be that little bit of competition between players that makes them work harder for the group. Without it you lose an important team mechanic, and chance for character growth.
My sheet always goes with me to prevent someone from tampering with the work I have put into my character and altering it. To me it isn't just a bunch of numbers on a page, but a persona that I want to act in a certain way and do things in a certain way. If I am ever absent the DM has always dropped in a stand-in NPC for the group if they needed it. Most times other don't even want to try to play my characters because of the personalities they have, they don't want to try to match them, and would rather me play them so the characters can entertain them.
So I will really matter what you are playing for. That said, everyone does not need to have the exact number of XP or even be the same level to maintain game balance and fairness.
4th has the function to scale encounters that can even be done on the fly after one has been made to adjust for the missing man or even an added party member.
I would hope the DM does adjust up when a few new players may show up, which means no reason for them not to scale down when one or two may be missing.
I see it as a problem that didn't exist except in peoples minds and the rules can never fix that, they must do that themselves.