...actually, I don't know. When I started, I was told we don't use it; I don't remember whether I was told a reason. If I was, it was performance; anything else would have been surprising enough to remember.
I wonder if it's similar to why Jane St doesn't make a lot of usage of the O in Ocaml (from what I've gathered at least, I don't work there.....yet). Object systems can be nice but they tend to make understanding your code all the harder since you have all the dispatching. I don't know anything about CLOS but this is pointed out as a reason against using Java and the object system in Ocaml in the Caml Trading video.