Having a unified file manager is kinda a good thing - otherwise every emu porter, etc, has to reinvent the wheel or copy someone else's code... a single menu can be improved and made more usable without all of the other devs needing to update their code.
As a side effect, this will encourage programs to support command line options and filenames, which will make scripting more useful (I'm thinking mostly background music players and things of that nature).
I guess in general, modularity is nice because it gives more options to the user - if there's a few menu programs around, people can choose the one they like best, and they can avoid ugly/difficult to use file selectors (if anyone's tried using the file selection box built into xmms in linux, you'll see my point...)