Is heating feature in an air conditioner bad?
It's not clear if you're talking about mini-split AC systems with a heat pump or window AC units with resistive heating. If you mean the former, those systems are becoming more popular and are a very efficient way to heat your house, especially if your climate stays within the external temperature range supported by the unit (down to 20 or 30 ºF in older units, down to 0 ºF or less in newer ones).

On the other hand the window units with resistive heating is probably not worth your time, they are inefficient and you are probably better of removing the unit from the window when it gets cold and getting a space heater.