There is a very good use for them: their existence proves that the distribution is uniform. Anomalies in the results would indicate that something is wrong (like, for example, very difficult to notice bugs in the code, resulting in incorrect hashes).

Also, I don't see the point of saving keys that have some 0-bits prefix. It saves some kernel instructions to simply skip that check, and use the hash target itself as PoW evidence.

Now - since I paid a few bucks to scan 3300 trillion keys, why would I ever share the private keys? It's like everyone will know in 3 seconds all the intervals that were scanned (or maybe it was just one huge continuous chunk? you'll never know!), so they can skip them. Unless I'm self-mutilating, to have found the correct key and not claim the prize just to make fun of others to look in the wrong places.

If I'd go the way of scanning keys at random, I wouldn't care at all whether I shared the keys. However that approach will never allow scanning 3300 trillion keys on a Thursday.

That being said, my biggest issue with Puzzle 69, 71, etc. is that it is impossible to validate that the GPU computes ALL the data correctly (what if a bit flips due to cosmic rays? impossible to know, unless each and every hash is verified); so if I'd be Bram, I wouldn't be so brave. I see too many points of failure.