Are they needed? I've just had to chisel the plastic bits off, whilst changing my radiator for a new one.
I don't want to do that again if my new rad ever has to come out, there does seem to be a lot of support for the rad without them.
I suppose the whole lot could bounce out of the mountings, but even with everything disconnected it took a lot of jiggling to get my old one out.
Bloody captive nuts, who designs these things....?