The big square ones are more associated with floorboards and the diamond ones with pegs, but it's not a hard and fast thing. Sometimes I think they just grab the next object in the bin and slap it on.
No, they come up from time to time -- you just need to watch for them. A lot of folks think the bar makes the bike ugly and remove/sell in pristine condition, or the bike is being parted out for some reason other than a crash and the bar comes up as perfect or slightly rashed but not bent. So it will be a crime of opportunity.