I am guilty of using the hairdryer once in awhile, I'll admit, but with my schedule and limited time, sometimes I have too.  When I do I always, always, always use it on the cool setting & I also use a long-fingered diffuser to help minimize the impact on my hair.  I also only use it to get part of the dampness out, never until complete dryness, and I always follow up with a leave in moisturizer like jojoba oil.  Since making the switch to cool air only, there is definitely a difference in the health of my hair, although I do strongly feel that letting hair air dry whenever possible is ultimately best.