Fixed-sized windows with native-sized content should be treated as maximized in certain instances
For instance, when using the features that hide titlebars+frames of windows when maximized. Fixed-sized windows with native-sized content is something that is very common with windowed games, so would be useful to have users don't have to manually center the game window.
Personally I wouldn't care if windows like this were just treated as always maximized. Although I could also see them treated this way by default but pulled out of it if moved, and then you can just re-maximize for it to re-center and get rid of the titlebars+frame.
EDIT: I forgot to say that there's also the issue of maximized apps with WINE not being able to utilize the entire screen (it has ghosting artifacts), but I'm not sure if that's the fault of WINE or xfwm4 or the XFCE panel etc. Or at least that was the issue, currently on my system it un-maximizes slightly.