An illegal element is an element, so unless it is performed after the maximum number of legal elements, doing a backflip means sacrificing the points for the last element in the program. Assuming that the last element is a spin, thapt could be three or four points in addition to the mandatory deduction and (as hanca suggested upthread) lower PCS. (And it's difficult to add in an extra, illegal element given all the technical demands these days). So it's really more like sacrificing five or six points instead of just one. It's not worth the risk unless, like Surya Bonaly at Nagano, you're planning to retire with a bang.