I can think of a couple of reasons: 1) the rear cap of my MH sits about 6 inches off of the rear wall. There's a bunch of heating ducts in there. 2) I think the dirtiest part of my MH, after driving in the rain/wet roads is the back cap from the down wash. You would have to clean a rear window all the time to be able to see out!!
Fran, Mary & Zoey (silver Cocker)
2006 Itasca Sunrise 35A, 2005 W-22, Allison 1000 5 speed
'13 Ford Focus ST or '10 Prius on Master Tow Dolly 80THD-SB or '00 Jeep Cherokee 4-down