The first time I tried a roll on a hard surface I seperated my shoulder...from kneeling. I can do them now if I have to, but like John Riggs have no intention of doing them just for fun. We don't practice them in our dojo but I and some of the other students have related stories of slips and falls we've taken in the "world" and our normal, matted, rolling/falling practice served us just fine.
Hell, one of the older judo guys I know was coming down a ladder from his roof and slipped off about halfway down. He landed in a back breakfall. Knocked his wind out and he was sore for few days but no other injuries...and they don't practice falling off 8ft ladders
I've done demonstrations with my sensei and we normally don't take mats and nobody seems to have a problem with the falls, but then again we're one of those wimpy Ki Society offshoots