I was keeping at least one copy of each SU catalog since I first discovered the company (I think the oldest was from 1998!), until a friend came and helped me clean out my incredibly messy and cluttered craft space. She, the voice of reason, questioned why I'd even need some of the older catalogs. At first, I was shocked she'd even suggest getting rid of them, but then I finally realized she was right. We looked through them one last time, laughed a little, and pitched 'em (in my recycling bin). If I really need info about a stamp set, I figure I can come here and ask, no need to hang on to old, dusty, dated catalogs just in case.
Oh, and I wondered if I should try giving them away or selling them, but decided it really wasn't worth the time or effort to me.
If you don't have the space, and aren't completely attached to them, it's probably time to part with them.