Libraries are different - here's one pole:
I'm a librarian at the Allen County Public Library system in Fort Wayne, Indiana. We have fourteen locations. Our holdings include over a hundred RPG titles - largely they are 3rd and 4th edition D&D, but there are also many d20/OGL titles, some Pathfinder, some Star Wars (WotC only), some GURPS, some Traveller (of the Mongoose variety), d20 Modern, Buffy tVS, LotR, superheroes of various stripes, World of Darkness, Shadowrun, Lone Wolf, Eclipse Phase, Serenity, and ... um ... TriStat dX.
That's titles. In many cases, especially with D&D, there's more than one copy. Is theft a problem? Yes, but not an insurmountable one. Looking at the 3.5 Player's Handbook, for example, seven copies have been stolen (or lost or missing), but about the same number have been discarded due to wear and tear, and about the same number are still in the system.
The way we see the read-fiction-once vs. the you're-never-really-finished-with-a-rulebook question is that that's a comparison that doesn't make sense. RPG rulebooks are really more like auto repair manuals (which we also collect in large numbers of titles and copies). They meet an information need. (A side issue: auto repair manuals also disappear, but not as often as RPGs.)
So you go to the library, check out the DM's Guide, use it for three weeks, and then if no one has it on hold, you renew it for three more weeks - up to 5 renewals, plus the inital checkout, and you might have a book for over 4 months.
Gaming is important here. I run a campaign for teens every summer (I work in the Young Adults' Services department, meaning teens). Sometimes I use an established game, such as when 4th Ed. came out, but most of the time I poll the likely players about a setting and then choose a rule set that makes sense. I've often used Dogs in the Vineyard (that's one we don't own) mechanics. By being proactive in this way, I establish my department (and by extension my library) as RPG-friendly. I have a lot of support in this, from other librarians on up to the director - we have several RPG-playing librarians on staff, and another librarian with a published board game credit. We also run a chess club, a go club, and miniatures painting events. And then there are the video games - one branch has collected them for many years, and all locations have had them for several years.
So I think it's important to say that we don't just collect RPG books - we're committed to gaming in a lot of different ways, and collecting RPG books is certainly a strong point within that commitment.
Ian McKinney
Assistant Manager of Young Adults' Services
Allen County Public Library
Fort Wayne, Indiana