I'd like to add that, using the ipad, I don't think it needs a case for protection except maybe against scratches. My iPhone has been naked except for a decal on the back since the day I got it, and it's seen worse care than my ipad ever will (simply because the ipad won't fit in my pocket)... and it still works fine.
Screen protection would be a plus... The glass is very scratch resistant, but if you drop it on asphalt, it's getting scratched.
But yes, if you want protection for drops, the Apple case isn't going to provide it. And any case at provides protection is going to be thicker... The apple case is very thin and will protect the screen from whatever grit accumulates in my bag... For drops, I'll just count on the protection provided by the glass and the body.