I don't know what ROKU did with their USB support. However, as an end user, I would be somewhat concerned if ROKU gave open access so channels could write/delete any file on the USB disk. I can see the benefit to having additional "buffer space", however I would prefer it if the channels don't have the ability to overwrite and/or delete data I may already have on the USB disk (so if ROKU does give USB write ability, I hope they limit it so that a channel can only write and/or delete its own files, and not alter any other files).
NOTE: Just because we don't have a "rogue channel" yet (at least that I'm aware of), doesn't mean that some malware writer won't come up with some "trojan horse" channel at some point in the future. And it would be a royal headache, if some channel starts deleting (or worse yet, replacing with SPAM files) data files on the USB disk. And even if you aren't talking about a malware writer putting SPAM on your USB, you still could have a legit channel with a bug in it, that causes files to be lost and/or corrupted (again, assuming that unlimited write to the USB disk is allowed by the channel API).
Now granted, as a precaution, I'll (personally) probably make a point of using a USB disk dedicated to just storing media I may want to play on the ROKU. And I'll probably have a "backup" of that media on some other disk (not attached to the ROKU), so I should be able to recover if/when some ROKU channel corrupts the USB disk's data. But I still would prefer it if a channel didn't have the option to corrupt (overwrite or delete) that data (on the USB disk) in the first place!