Cement resistors generate heat over a small area. So you would need to use either a very large number of them, or use aluminium heat spreaders to spread the heat from each resistor out over the zone you want it to heat. You would also need to use borosilicate glass instead of float glass to avoid the risk of cracking when you only heat part of it, and you would need independent temperature control for each heated zone in order to get an even temperature.
Not worth the extra cost and complexity IMO, except perhaps if you have a very large heated bed. If you want to save energy, I think a better option would be to enclose the printer to keep most of the heat in, thereby providing a heated chamber as well - which is needed when doing large ABS prints anyway.
Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]
Whilst I agree with most of what dc42 says, I wonder about "you would need independent temperature control for each heated zone in order to get an even temperature". It seems to me that having a single thermistor and assuming that the whole surface is evenly heated is exactly what most heat beds do. I don't see that having (say) 4 small pads instead of 1 large one will be significantly different.