You will find that many automatic customers will stop and use the towel station once they learn about it-probably 25%. Even if the car is mostly dry they will wipe streaks coming out of mirror housings, etc. Many SS customers also use the towel station. So, as you can imagine this cuts into towel sales. But I still sell some towels. Abuse of the service seems to be the biggest problem overnight. I originally brought the towel cart in overnight, but now I just leave it out with fewer towels.
Another idea I have heard of (if you are well attended) is a "clean towel club" The customer buys one nice terry towel for a nominal fee and it may be re-exchanged for a clean towel whenever they visit.