if you cut the ac belt the only thing it will affect is the power steering
that's assuming the engine has two belts, the other belt will work with the water pump and alternator
***warning*** if the engine has only one belt you cant cut it
the defroster and heater can be affected because the ac compressor work with the system but if you dont uses the compressor with the heater it will work good but not as is suposse to work