You're right! This is a lot of hot air for such a minor thing. I suppose I could be accused of being biased since I make a lot of money servicing regulators that have had a few drops of salt water in them.
Two things however have not yet been discussed.
1. There are a dozen different dust caps and some are more likely to be a problem than others. Even the most common cone-shaped style comes in several versions. The first thing you need to do is ensure that your dust cap does not trap much water. Quite a few have a large hole inside right where it fits over the regulator filter which might hold as much as 1/2 ounce of salt water. It's also very difficult to get all the water out of that type. The easiest to clean are the hard rubber balls. So first you need to have a good dust cap with a flat, easy-to-clean surface.
BTW, I don't tell my students to remove the cap during a dive. I don't believe it's necessary; it's easier to lose in your dive bag; while looking for it or getting it out of your bag, salt water can drip or roll into the reg; and many are attached to the regulator by a thick rubber strap that requires you to remove the yoke screw to take the cap off.
2. Trying to dry the cap with the tank air is annoying at best and inefficient at worst. As stated above, unless you are very careful, you may actually blow sea water into the reg. I also can't see the towel drying as practical. I want to dry the dust cap immediately after I take the reg off - not hold it in my hand while I fish around for a dry towel (on a dive boat yet!). Why not hold the cap in your hand, look at it to ensure you are blowing at the right side and blow on it until it is dry and free of residue? Yes - blow on it with your mouth. It's quiet, controlled, warm, dry, cheap and you can watch the entire process so the cap gets dry and the reg stays protected.
