In support of the priority feature, SelectCoins only uses your own 0 conf transactions only as a last resort if that's all you have left. This helps keep you from turning your coins over rapidly unless you're forcing it by actually turning all your coins over rapidly.
Of course, if the network is not being flooded and you're not overly concerned about the current transaction getting held up then it's probably worth preferring to use your 0 conf transactions so that you can "save" the higher priority coins for when the network
is being flooded.
Unless I misunderstand, it looks to me like the current logic is more likely to spend the accumulated priority of older transactions. It's a minor point however.
Gaming the system by including 1000 or so recently turned over BTC to bump the priority as described in my post above still works of course!
ByteCoin