Like most raiders, my guildies and I occasionally stare down a raid night where we know several people can't make it. Schedules change, real life beckons and we find ourselves with a raid that's down two to five people and wondering: a.) whether it's worth it to cancel, and b.) at what point you should throw in the towel. For better or worse, the normal/heroic mechanic in Icecrown Citadel has added a new wrinkle to the issue, too.
It's easy to cancel when you know for sure that there's no way it's going to happen (e.g., if you're looking at a half-empty raid, or the heal team has decided to play hooky en masse), but for us the trouble is usually in that ill-defined area between what an undermanned raid can do and what everyone would rather be doing. If you're missing a few DPS for a normal ICC clear, that's not a big deal, particularly if you're using the 10% damage/healing/health buff. However, not having those DPS for heroic attempts (especially for tougher heroic bosses like Sindragosa and Putricide) is basically a death sentence at this point. The question then becomes, is it worth it to clear the content on normal so the night's not a total waste, or do you cancel the raid, bank on better attendance the following night and hold out for a kill that may not even happen?