I can actually see more of what's behind me at a much higher resolution than the OEM camera although I don't use it in that manner. If I wanted to I could easily wire it to come on anytime the car is in reverse.
Generally a dash cam has such a wide field of view that using it to backup, especially to get close to things, would be much harder. I suppose we can get used to anything, so it might still be better than what the car has now; but most the dash cams are close to 180Âş field of view, and I think most the reverse cameras are much narrower, 100-120Âş if I had to guess (I'm far from an expert, so I'm totally just guessing).
That said if you do decide to wire it into the reverse, I'm interested in how it goes. Could be a pretty slick mod.