well, just a few things that come to mind immediately...

- much more space for installed apps
- much more space for app data files, with no effort to set links or whatever to get them to store data on the external mmc card
- fallback to standard flash install if/when you do something to mess up the mmc install, and then hopefully be able to fix the mmc install (you'll need modules to mount ext2fs or whatever you use for filesystem on the mmc)
- remove the external mem card, boot to flash, and back up the complete mmc filesystem over usb (again you need to be able to mount ext2fs or whatever from pc, not a problem if you run linux), or backup using an external card reader
- variation on above, be able to get your app data off the mmc card if/when you screw up the install and it no longer boots and you didn't bother to backup
- (bootmenu, not mmc boot advantage) can now access the standard flash install filesystem over usb networking through new bootmenu, giving many of the same backup/recover advantages to the flash install as for mmc install above - and installing bootmenu is a prerequisite to booting off mmc

'much more space' above defined as your mmc card size minus whatever the standard flash mem is - 64mb on n770, 256mb on n800? I have an 8g sdhc card in the internal slot as my normal boot option, with the internal flash as the only alternative.

and yes it is a little bit faster booting, but as far as I can tell the normal IT use case is not supposed to involve regular rebooting.

rob.