I'm not sure about others who wanted a file manager, but for me, I can think of at least one specific situation: Downloading attachments in an email before saving them. Before you have the option to save an attachment of - say - a photo, you have to download it to the phone. THEN you save it, and the saved files go to the photo hub. The downloaded files that weren't saved to your photo hub take up space in Other and don't seem to go away, so with a file manager you could navigate over to a "Downloaded Temp files" folder and delete it.
Oh yeah, and the people who have had files transferred to their phone via NFC or Bluetooth or whatever and couldn't find them in any of their hubs (i.e. lost)... you could find them using a file manager.