Yup, and 10% is about the percentage of rpg players I'd have guessed bother to register at forums.
Anecdotally, in my group of players (10 persons) one person registered here (but so far never posted) and one sometimes visits the WotC forums (I don't know if he's actually registered or not). And that's after I continually encouraged everyone to join the forums!
Two of them are what I'd consider collectors, i.e. they buy (almost) every book. Myself and a third player buy about half the books that are published, and the rest only ever buys one or two books beside the PHB.
This is why I keep on saying that you have a few general groups of gamers: Casual, Serious, and Hardcore. Casual gamers make up somewhere around 90% of the total, maybe more; the bulk of them don't own any gaming books, except for maybe a
Player's Handbook. It is the Serious and Hardcore group that spends most of the money.
My experience is similar to yours in that of my group of six, only one person (myself, of course!) is a Serious-Hardcore gamer. Like you I buy about half the books; I don't think anyone else in the group has more than a
Player's Handbook (although they all bought it, at my request). They are also, to the person, the exact demographic that I was saying WotC should go after: All are in their mid-30s to early 40s, none of them have played D&D since college or before, and all were re-introduced by myself with this 4E campaign that we started in late 2008.
None, however, are likely to buy more than one or two books; one might buy a PHB 2 or 3 if they want to play a class, but many of them have purchased at least a month of D&D Insider and are happy enough with that for character making options. Which is an interesting phenomena: Not only is DDI stopping serious+ players from buying many books, but it is making it less necessary for casual gamers. Hmm...maybe another thread on this topic.
