Gloves are something I would ALWAYS suggest, not just for when it's a bit nippy.
If you really need to know why, just imagine what happens when you crash/fall off your bike, even as low as 20mph.
End of the day, your hands, your skin, your choice.
Heated grips serve their purpose well, but if it's so cold that you feel they are pointless, then it's probably too cold to ride anyway as the grips will still work fine but rest of you will freeze. Hypothermia is a serious risk to consider in Winter riding.
For the record, -7ºC with a 70mph wind = around -35ºC and is far too cold for any sane person....