Using a belt GREATly increases the risk of a hernia. This is because it builds up the pressure in the abdominal area, and all that pressure will look for a way out. So if you have any weak areas, bang, you're wearing your intestines on the outside.
Besides, as long as you are using proper form, a belt is not necessary. I have a bad back from years ago, but have squatted 445lbs for 5 reps, ass to the floor, and even deadlifted 550lbs for 5, all with no belt. And the flip side is, my lower back is just as strong as the rest of me, and my old injury feels great. If you use one for your regular working sets you'll end up with a lower back that is weaker than the rest of you, and that imbalance can easily lead to injury