If the 1 block old shares have any chance of resulting in a valid block (they do) they should be accepted! Rejecting them only makes profitability "look" higher while it is actually lower. You were the only pool doing this right don't be like the crap pools!
The problem is, there are times when it is possible, and there are times when it is not possible. The times where it is possible is _significantly_ less than the times that it isn't.
Coin1 --> Coin2 :: "1-stale" still possibly a block find
Coin1 --> Coin2, Coin1 moves to a new block :: "1-stale" not useful
Coin1 --> Coin1 (new block) :: "1-stale" not useful
Also, it depends a lot on the actual hashrates that come through. For example, if we allow "1-stale" shares, and say 1% of them would be valid blocks, thats awesome, we get 1% more blocks, hooray! However, if by accepting those "1-stale" shares, we end up having miners over-tuning their cards (because they get more shares) by 2% (
extremely conservative estimate), yeah, we're finding those 1% more blocks, but we're losing 2% of our overall power. Not only are we losing out overall as a pool, the people tuning their cards heavier are actually taking a larger slice of the pie than they should (they're contributing less valid shares, and getting more accepted).
I don't see a downside of rejecting 1-stale shares except for the negative of users getting pissed that their reject rate skyrockets overnight.