Everything also depends on the loads attached to the receptacle, in a house there will almost never be anything plugged in that would seriously strain a device, the obvious esceptions being airconditioners and space heaters. and if you work remodling houses you become very grateful for not having to jam a wirenut and extra wire into a box along with the old, stiff, rubber and cloth covered wire. Personally I think that the conection to the device is far, far more likely to fail then the device itself under load, especially if not properly installed.
I'd also like to say that back stabs may be a good thing for speed-freak tract home electricians, they do everything fast as possible, and if they had only screw terminals to hook to I can see such things as sticking unlooped wires under screws, not tightening screws down all the way etc.
-Will
