Yeah, from that point of view for a tick entry then `Show Message(s) Window` is better.
So the only issue is message/messages. Only one of the tabs in the message "Window" (actually its a pane, but lets not get technical :-) is called "Messages", there are also "Status", "Compiler", "Scribble", and "Terminal" as well. So really calling the menu entry the same as one of the tabs is not the right thing, it is not related to the particular tab, its all tabs.
Simply removing the `s` is the minimal menu difference unless somebody can think of a better name than "Message Window" or a better name for the "Messages" tab.
(All I could think of was "Miscellaneous Window", but no thats not better).