Turn the heat on and motor idles rough but seems to want to accelerate?

[Eg.h2o]

If the defrost is on, the A/C compressor will turn on regardless. All cars with A/C will do this if the defrost is on.

yes. This ^ ^ ^

if you have the blower set to blow on the windshield, the AC will come on to help remove moisture from the warm blowing air. This is to help keep the inside of the windshield fog free.