Mr. Spencer is old. And aesthetically displeasing. Holden doesn't really get past that, which means that we don't really get to, either. We're bombarded with images of white, hairless legs, a ratty bathrobe, pills, and the smell of Vicks Nose Drops, which again, is not so attractive. It's no surprise then, that Mr. Spencer lectures Holden, and is all around very un-hip.
He is, however, good for one thing: he introduces us to the way adults (or at the least authority figures) view Holden, and the way he views them in return. What we see is primarily a big lack of understanding. Mr. Spencer can't understand what's wrong with Holden, why he doesn't apply himself, or how to help him. Holden can't understand what's wrong with him either. The one thing he can be certain of is that he doesn't understand the appeal of the sort of life Mr. Spencer (and all un-cool authority figures everywhere) promotes, a life of office work and martinis and a certain number of miles to the gallon.