I think there's a useful distinctions to be made here. There are different types of rearers, IMO and IME -
First, a lot of horses at some point in their training will try a small rear as a resistance, a "No, I don't want to and you can't make me." or as part of a spook. I don't mind dealing with this; driving them forward and/or keeping them from planting their hinds works very well. If it's not a successful resistance, that is, they aren't able to avoid work by doing it, time and training pretty much make a non-issue.
Second type is a "confirmed" rearer, a horse that has been mishandled and allowed to be successful at rearing to avoid work and will rear at anytime they're faced with something they don't want to do. IMO, these are VERY different problem and very hard to fix. When I was younger, fitter and stupider I did work with a couple and work throught it, but they were never horses that could be trusted with a tentative rider. Foxhunter's story about standing in the cross roads for 3 hours sounds like a confirmed rearer.
Worst of all is a confirmed rearer that has flipped over and still rears. This, IMO, is a dangerous horse, one that cares more about resisting that its own safety. This horses should not be ridden and should probably be put down. (It's dangerous even for a handler on the ground, I had one flip over in a narrow barn aisle right next to me once