A couple days ago, Ze Bullette posted a poll about what GMs do when a player can't make the session.
It reminded me of a time, back with my Evansville Group, when one guy was constantly unable to attend. We passed his character--the Cleric--around, taking turns running him.
When it was my turn, we got into a melee with some goblins or some other humanoid opponents. We're surrounded, and everyone's toe to toe with opponents. I'm running my Dwarf Fighter/Thief (#3 in the linked post) and managing not to make any of the natural 1 rolls that seemed to plague me with that character, and also the Cleric.
Another character drops to negative hit points, but as I said both my Dwarf and the Cleric are toe to toe with opponents. On my next turn, of course I have my Dwarf continue his attack, but the Cleric also. I thought it just made sense to take out the current opponent first before turning my back to it, risking this other guy's character getting shanked in the back with a parting shot, when the downed guy could be tended to in the next round with a Cure Light Wounds.
I got slapped with an XP penalty to the Dwarf by the DM for 'not playing the Cleric right.'
Never quite sat well with me.
The DM, otherwise a pretty fair guy when applying the rules, thought a Cleric by default should jump to heal anyone in the party the instant they needed it. I thought, as a rational human in combat, you should never turn your back on a foe if you can help it. But he was the DM so his interpretation was the one we went with.
So I've ended up favoring the option of just ignoring the absence of a character if the player is absent, and ignoring their sudden return when they come back. I'm playing a game with my friends, not writing some sort of collaborative work of fiction intended to entertain others.