Or rather, why I'm not advocating that we establish a presence there.
I've been a user of SL for a year and a half now, before the big media blitz hit. I love SL. It's fun. I have a little SL shop where I sell stuff I've made, and it pays for my premium account and a tiny slice of virtual land that a friend and I own together. I socialize. I listen to music. I DJ once in a while.
I completely "get" why people are excited about SL and want to set up library services there. I was a member of the SL Library 2.0 project when it was first starting up. I've taught SL library classes (photos of my avatar doing so have turned up in more than one conference presentation).
When colleagues ask me about SL, I'm happy to tell them what I know about it. I also give them two big reasons why I don't want to spend time and effort setting up SL services for our library.
First, our students by and large aren't there. SL isn't much of a blip on the radar of the typical undergraduate college student. The average age of SL users is mid-30s. Everyone I know on SL is at least in their early 30s, often significantly older. A colleague recently told me about a conference session where the presenter had asked a room full of students if they used SL and got a lot of blank looks in return. I think people unfamiliar with SL see it, hear about it, and think "Second Life looks like a video game," so they assume student-age users. (There's another erroneous assumption in there: video gamer=teenager. Maybe I'm wrong.)
Second, I don't think there's a single computer in our library that will actually run Second Life. I think that's a barrier for a lot of people who might be interested in educational applications of SL -- unless you are a gamer, you're unlikely to have the hardware to run it. Academic libraries, in my experience, tend not to buy computers with 3d graphics cards.