I think the system is smarter than you expect. I leave the AC on all the time, and have not noticed any drag on the engine due to the compressor when cooling was not required (like when outside temp was below my set temp).I've only had my MC since October so have had no use of A/C. However, when I first got the car the A/C light was alway on with the heat running. I kept waiting for it to cycle off, but it never did. So, at least in cold weather, the system doesn't seem smart enough to turn the A/C on and off, it's just where you put it. Or I'm not smart enough to understand how it works, even though it has been much discussed on these forums.
I was just going to add this advice.It might be outdated advice but regardless of whether or not you routinely have the a/c on I thought it was a good idea to run it at least once a week, preferably on the coolest setting, to keep the refrigerant circulating, stop the seals driving out, and nasty wiffs developing?
As Jeremy says.........usually only a chilly inconvenience in the UK in winter but maybe more challenging elsewhere.Yes you have to run it on max coldness I think but not sure what happens if the external temperature is colder!