Pidgin on Windows, for one. And even on other platforms you have to compile and install the third-party plugin for it yourself, it doesn’t ship with Pidgin - this is beyond what I would expect most users to do. (I imagine the same is true for Adium as they’re both libpurple clients, but I don’t have a Mac handy to check.)
Those two alone are the clients that most Windows/Mac XMPP users I know reach for first, probably out of familiarity.
Those two alone are the clients that most Windows/Mac XMPP users I know reach for first, probably out of familiarity.