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?
For us, the answer is usually the latter, and I'm wondering how other people deal with this (apart from recruiting for a bigger bench). While attendance has been always been a thorn in raid leaders' sides, the normal/heroic mechanic in Icecrown content's resulted in the seeming absurdity of postponing a raid when we're missing as few as two heavyweight DPS. Nobody likes to see a raid night wasted, but it's also no fun to look at that Deathbringer's Will that just dropped and think to yourself in agony, "Maybe if we'd waited a day, it could have been the heroic version."