Not sure about the depreciation thing. Maybe it's like luxury cars - they don't seem to hold their value as well as mid-priced models for some reason.
As for why to choose a Class A, I'd say two reasons - comfort and storage. I started with a Class C. After 5 seasons, I moved up to an older Class A gasser with no slides. There was more room but all the basement storage was the really big advantage for us. 5 more seasons and we moved up to our current rig, a 37 ft Class A gasser with 2 slides. Slides make a much greater improvement to comfort than you might expect. And, this one has even more storage.