I have done work with and without belts. I honestly don't view a belt as so much of a tool to keep the back straight, I mean...you still need good form and you can arch your back with one too. I've seen a lot of people with arched backs and belts. It doesn't necessarily increase stability in that way, IMO.
What I see a belt as being good for is for keeping your stomach from pressing out during a movement. This is easy to do, say you are half way up with something heavy and have to take a breath and just lose it for one second or something - bang, instant hernia because something went wrong. It shouldn't be used as a crutch or an excuse not to learn proper form, but I also use a power cage when I squat and don't squat without properly positioned safety bars. Would some guys just drop the bar and such? Sure - but I'm not a competitive athlete, I am a guy who has to work for a living and wants to be healthier, not beat up and broken from trying to be tough or pretend I'm never going to make a mistake.
Having said that I'm pretty limited on my belt use. I don't normally use it for squats. I personally have a hard time pushing my stomach out with the bar on my back, my entire core becomes too tight. But I will typically do warm ups with maybe 225 on the deadlifts with no belt. Once I start doing working sets though, the belt goes on. I never use my belt as a means of keeping my back straight, like doing militaries or something for instance. Seems to me like that is a form issue that needs to be corrected.