I know of no MMO that officially tolerates mods, as in client modifications. People do it anyway, as the risk of being banned is very minor to nonexistent unless you are dumb enough to use them to cheat in an obvious manner. Personally, I love my mods that remove the dumb as fuck glued on shoes from my Kitten. I like to be barefooted now and again, even if the Japanese consider that pornographic. :'D
Do keep in mind though, that the person you quoted did not refer to client modification but rather to addons such as recount etc.