I've heard of player entitlement, but this takes the cake. "Hey, I made a character, you have to run for me, even if the other characters don't want to associate with him!"
That's certainly how I want to spend my limited time...
Never said anything about "have to run", only that having a PC acting alone gives the DM some options. All I'd expect, were Harry my character and were I a continuing player in that game, is every few months while waiting for other players to show up the DM and I roll a few quick dice to see how he's doing - is he dead yet? Or rich? Still adventuring? Retired?
That way, if a situation arises where Harry somehow becomes relevant again he's already updated, and the main game doesn't have to grind to a halt while we update him then and there.
I hit this all the time, not so much from characters being thrown out of parties but from being voluntarily pulled out by their players. An example: mid-adventure Joe's character Grazzle dies, leaving Joe with nothing to play for several sessions until a revival can be obtained. This isn't any fun for Joe, so he rolls up Frazzle as a short-term replacement and the DM finds a way to bring him in. At adventure's end Grazzle gets revived, and Joe - who would rather play Grazzle than Frazzle - retires Frazzle.
But Frazzle doesn't disappear, and he's still Joe's character; and so it's incumbent on Joe and the DM to every now and then update Frazzle even if said update consists only of:
Joe: Frazzle's just gonna sit tight in Brighton and live the pleasurable life.
DM: <rolls a few dice> OK, Frazzle's still in Brighton. Nothing of note has happened to him in the 4 months since he got there, and he's the same level etc. as when you last played him.
Now if the situation arises later in the campaign where Joe loses Grazzle for good and wants to bring Frazzle back in instead of rolling up something new, that's a bit more trouble. In theory, by now the party will be a few (or more) levels higher than when Frazzle last ran with them, thus if Frazzle wants to have also (maybe) gained those levels we need to figure out how he did it. For this I've developed a set of guidelines for what I call "mini-dungeoning", where with input from the player whole adventures are condensed down to just a few dice rolls. With this I can update a long-retired character in a very short time, should the need arise. (or kill it off: mini-dungeons are about as risky as normal dungeons and I have lots of data to give me the odds)
Lanefan