> Easier is just to prevent bad data in a filesystem from causing a panic,
> or other crash, regardless of who mounted it, isn't it?

Yes. It depends on how much trouble it takes to implement that safeguard. 
For non-root, its a must-have not to let bad media crash the system.

  | Maybe, at mount time - a check can be made by the mount command if it 
is 
  | safe to mount a filesystem if an unprivileged user is mounting one.


[snip]
>wouldn't provide any protection against bad data growing in the 
filesystem
>due to I/O errors, etc.

Yeah - this is something I overlooked when suggesting the above. 

regards
-kamal