I have been hearing this NONSENSE forever...By now I would have known if there was a issue doing this...I left the a/c on in my 88 Town on for 11 years and 300K miles with no issues whatsoever...Same thing with my 99 Grand Maquis [7 years and 200K miles] and the rest of my cars...They are left on 365 days a year...In fact I left the the a/c on all the time in my big boats in the 70's all the time...Just left them all on in the automatic position...The cars we have now have been left on since the day we got them.
If anything it's the people that shut off the a/c everytime they get out of the car and put the a/c back on everytime they get back into the car are the only ones that seem to have issues...BTW we have no smelly or mold issues.