Most importantly, we have to remember that it is not all about search.
If a page makes my site better but is never an entry-point, it's valuable and I'm going to leave it. If you have 3,600 pages on your site, a good chunk of those are going to fall into that category. Sometimes you have something important to say, but nobody is looking for it. And sometimes you have content that was once topical (and still valuable, like an old news story), but doesn't gather a lot of interest anymore. I'd leave that too.
I guess the only time I would remove content would be if the focus of my site shifted and it no longer fit with the theme, or if I had been hit by Panda and had some low-quality content that needed to be addressed.