One advantage I see, is non intrusiveness of the WM in your other apps workflow. For example, let's say you use ALT+[0-9] to switch between channels in irssi.

With a "normal" WM, if you bind ALT+[0-9] to switch the current workspace, you'll not be able to switch channels in irssi anymore, but with a modal WM, if you're not in "Workspace management" mode, ALT+[0-9] will have no effect.

So the biggest advantage I see, is that you only have to worry about one keybind: the mode switcher (but I might be wrong... That's how I see the modal approach, though)