I own and use systemless setting books, most notably, the systemless Pirate's Guide to Freeport that came out in the 3E/4E transition era. For the most part, such books created for roleplaying purposes have clear analogues in the most typical rulebooks anyway (if there's an animated skeleton in the setting book, I'm pretty sure I know where to find it).
I often change ignore the stats in the rulebooks that have them anyway to scale up or scale down challenges as I see fit, so having the numbers be a loose guideline to begin with aren't an issue for me at all.
On a related note, for the Alien movie franchise, there are a lot of field guide and blueprint books that I think of as being ideal accompaniments to the Alien RPG, even if not everything in it shows up in the RPG with stats. Just knowing that there's a gun out there that's slightly different in X or Y ways, or the ship the PCs are on is this big and has those features is almost as good as having full stats to begin with. (In the Alien RPG, I'm not likely to have a lot of ship-to-ship combat to begin with.)