I keep old catalogs (I have a couple CTMH and lots of SU). I still pull out old catalogs when I'm trying to come up with a card or scrapbook page. Even if the stamp images or papers are outdated, I often see design ideas or layouts that are perfect to use with my current stuff.
Magazine I don't hold onto as much. I'm more inclined to rip out pages/pictures of projects that interest me and put them into a 3-ring binder and not keep the other ads/article/pictures that don't interest me.
If you are conflicted, I'd say hold on to them in the attic and after a while, pull them out and flip through them. If they inspire you then, keep them! If not, then think about donating them.