Considering that the wheels are the first thing to get dirty on the car, I've never once had a situation where my car needed washing, but my wheels didn't.... So I totally do not "get" the method to that madness...
Furthermore, I always let the rotors air-dry just standing still... Makes far less mess than going out sopping-wet and slinging brake-dust-water-slurry EVERYWHERE... Just don't drive the thing for an hour after washing, and it'll all dry-up, and the slight amount of surface-rust will just go "poof" into the air (for the most part) when you go driving... I've tried all methods, and the least-labor-intensive method is just letting the car dry fully prior to driving...
Where your RV covers DO make sense, is to keep water out of the rotors, and then go in and hand-wash the wheels with a damp rag and towel, no hosing, to keep the rotors dry... That indeed would work just dandy...
Or, of course, you could just forget the dang covers, and be careful with your hose not to hit the wheels, and hand-wash the wheels, and you'd be just as good. It depends on how crusty your wheels have gotten. I never let mine get bad, so just a gentle swab with a soft wet towal cleans them up nicely.
Who cares if your car is dirty?? Because it looks better clean! And it's an issue of self-pride, just like you don't go walking around in a perpetual stench day after day, with dirty underwear and clothes you don't wash until they crawl themselves into the machine!! A clean car is a reflection upon your own housekeeping, a sign that you at least make the occassional effort to maintain your fine piece of machinery...
Another in a series of posts railing against our ever-worsening "disposable consumption" society, harkening back to the days where money was hard-earned, and the fruits of your labors were well-cared for regardless of how spirited the use.