Weightlifing belts do not as much support the lower back as they increase your intra-abdominal pressure. The best support you can get for your lower back is strong abs, strong spinae erector, and good lifting form. By increasing intra-abdominal pressure belts will allow you to lift heavier weights, but they will not stop an injury if you use bad form or have an imbalance between your abs and your spinae erector.
Just to second the injury part. A couple of months ago I injured my self doing DL's. I tried wearing a belt( something I don't normally due) I think I put the belt on to tight causing my ab muscles not to fully contract and tearing or pulling the ligaments in between my pelvic bones.