2% if your using it? Are you serious? Go throw on your air conditioning and try flooring it from the lights. You'll find more than 2% power drain to the compressor!
Yes they have a clutch, however have you seen the wobble plate and control valve in them? This regulates the compressor. Why would you have this setup if you needed it to perform in a high drain situation and off only? Instead the compressors work in a high drain, low drain (no a/c on) and off mode. As to when it's actually used i couldn't say. But the compressor does vent gases into the crankcase.