"Single menu means a single point of failure as well. If something happens to that menu, you're potentially screwed unless you have a shell open. Many times on OSX I've had a program go into "beachball of boredom" mode, and when that happens, quite often it takes the menu with it. I can switch to emacs or bash and get some work done, but pretty much everything else relies on the menu not being wedged, and so is useless until the rogue program gives control back."
That's not a problem on Linux with X and KDE. The menubar is in a separate process and X and the window manager control which app has focus. Apps can virtually misbehave as much as they want, but X and the window manager still control the screen. The single-menubar shows the menu for whatever app the window manager gives focus too, even if some apps are frozen.
Apple just has a poor implementation...even after all these years.