I've owned five T3's at one time or another. One of them was a no-ad back. I wasn't looking for a no-ad back - I actually bought it because it was a Mullin and really wasn't all that interested in the no-ad part of the card.
Obviously that's completely anecdotal and not conclusive, but it would seem odd that I could fall into something like that if it were truly rare. Possible, of course, but odd. I think they are out there in not-impossible numbers.
I think people that watch these backs closely might find them more uncommon, especially if they are looking for a specific player with the back. That might take a very long time to find.
To the extent that these backs are of passing interest to the hobby (meaning that most hobbyists are aware of but not really chasing them from a collection standpoint), they do carry a smallish premium. Although in my case, it was a one-way premium - the premium was there when I bought it but not when I sold it. Vanished into thin air, it did!
Joann