- people having rare/strange/broad fields of interest cannot count on having a whole bunch of other people sharing said interests at least partly, to live around the corner (actually I learned to keep my expectations low - I'm glad when other humans I have to deal with are capable of autonomous thinking)
- development, learning and cultivating new ideas requires exchange. If we exclude "date and mate" forums from current observation, physical presence is at no point of the contact nesessary, so you can trade information at very low transaction costs, because "going where alikes are" means to find the right forums.
- newsgroups did and do basically the same, only using mail protocols for that. the only difference is, web-forums are fancier, faster easier to operate, but - without the content synchro of different threads like among newsservers - more locally isolated. The basic idea behind both concepts is pretty close, so we can surely say, what people do browsing webforums, has a long tradition and was since internets childhood one of the most valued and rewarding uses of this technology.
Keeping all this in mind, I believe being a member on a handful of boards is far more normal, than being on none.