I too thought that if posts were open ended that it would hurt the daily pool available for voting.
But then I realized: Oh shit - no it doesn't - because everyone is now eligible for open ended voting on posts...and so there's no loss because everybody wins.
If anything it's just a +1 improvement for everyone :)
I think there is more to it than this. What happens if you have received rewards for a payout, then remove your vote and vote for something again? you could earn rewards over and over.
A decaying reward pool might be a valid option though, but would need to ask some one with more technical knowledge than myself. I wonder if @ned has ever addressed this?