If you up and leave in the middle of a raid, chances are you'll derail the raid for everyone else, too. Obviously it depends on how many people are in the raid, the difficulty of the content, and whether there are any replacements available. Still, it's often the case that if you leave, you screw over a bunch of people. Most decent people don't want to do that. Even if you don't care, though, servers are small places, and the number of raid-capable folks is smaller still. It doesn't take long to build up a reputation that will keep you out of most end-game content.