Regarding cleaning the inside/backside of wheels: Even if you don't care how they look when you peer through the spokes (can't imagine somebody not caring, but anyhow..), there is a *functional* aspect to keeping them clean- if you have your wheels balanced with adhesive weights (as IMO you oughta) and especially if you only have the weights mounted on the back side where they don't show (gotta go about this right but it's how I have it done), then you want a nice surface so the weights adhere properly. Also, a smooth, slick (as in waxed/sealed) surface will prevent the build up of [stuff] that can throw the wheels out of balance. This is a bid issue with certain SUV wheels- people let mud/etc. build up in there and then take it in with complaints that the steering wheel vibrates ("no sir, there's no problem, except that your disgustingly filthy wheels were caked with stuff that you should've cleaned off...we did it for you, took over an hour: $125 please..").
Re cleaning the back sides of tires: Ever grab hold of a never-cleaned tire? Like maybe when changing a flat? Yuck, even if you're wearing gloves you're likely to get dirty. And it's a lot easier to mark tires with grease pencils/etc. (for rotation notes) if the sidewall has never been really filthy for a long time.
Cleaning the treads? Usually only for shows or photos.
Oh, and "pitting" in this context is corrosion of the wheels resulting in, well "pits" in /pinpoint erosion of the aluminum surface. Most wheels aren't finished all that well on the back side and will corrode, sometimes severely, if you don't take care of them.
