Tortoise:
We rarely use cohorts at this point, but that's a suggestion which hadn't really occured to me and I'm certain will provide a workable solution. Thanks for the input

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Setanta:
I suppose you're right about not making a big deal regarding the realism of characters popping into and out of the campaign...it's a very viable means of handling no shows and absenteeism. This also has the additional benefit of not trashing existing characters entirely.
Garmorn:
We already rotate adventures and campaigns, and none of the regular players really want to spend our valuable time on several months of "goofy" sessions. I'm not trying to denegrate the idea of an occasional comedic or hack & slash one shot adventure, but realistically we're talking about four or five months of gaming here. I appreciate the suggestion, but none of the people in our group can afford to spend six hours every other week wasting time on a frivolous campaign. Once in a blue moon is fine, but the time we invest in D&D is taken pretty seriously given our schedules and RL commitments.
arcady:
I understand the point you're trying to make, and to a degree it's quite valid. Mature adults should know their schedules and availability; make plans accordingly; and then live up to their commitments. A lack of this kind of behavior is part of our problem right now, and it needs to be corrected. But moreso, we're seeing one or two of the players placing D&D as lower priority right now than they generally do during the colder months. This needs to be resolved, and perhaps using a cohort/NPC system is the way to go.
Just don't generalize too far, though, because as Setanta points out there are plenty of very good reasons to cancel or miss a session on short notice. In those instances we're plenty lenient and understanding, but they comprise a small portion of my group's present situation.