I can't think of a good reason. Nick Craver♦ commented:
Say we allowed this, wouldn't that also allow a fairly unthrottled way of transferring rep?
And the answer to that is yes. However, the current system is lightly throttled as fredley comments:
As it is it's fairly unthrottled. I can transfer 3*Max bounty per day, which is a lot of rep.
In addition, the whole point of this bounty reason is to transfer reputation (as opposed to the other reasons which also are interested in getting new or improved answers). Finally, the system has already been adjusted to prevent certain types of gamesmanship:
- To discourage overly promotional bounties, if you are starting a bounty on a question you yourself have answered, the minimum rep cost is 100, increased from the standard 50.
- To prevent “infinite” bounty periods, multiple bounties started on the same question by the same user double in cost every time. So if the first bounty is 50 rep, the next will be 100, then 200, then 400, then the maximum.
But I'm not interesting in promoting my question, just rewarding a really good answer that I didn't happen to accept. Waiting 24 hours only gives people besides the person I'm rewarding the false impression I might award them the bounty instead.
If preventing someone from draining their reputation in less than 24 hours is the concern, why not continue to limit the number of bounties offered each day to 3 and remove the waiting period for awarding bounties? (Perhaps the waiting period is not needed at all?)
For reference, the dialogue box item in question currently reads:
Reward existing answer
One or more of the answers is exemplary and worthy of an additional bounty.
My reading of this statement is that by taking this reason, you are signalling that you want to transfer reputation to the user who wrote one of the existing answers. To me, the "or more" is either a pleasantry or an indication that the person offering the bounty has the option to reward multiple answers. (The second is not strictly true, since there is a waiting period, but in practice one can easily accomplish it.) In no way do these words imply that the person offering the bounty is seeking more answers. In fact, the opposite seems true to me.
According to Jeff's answer to this question:
If you start a bounty, part of your contract with the community is to allow everyone to have a shot at potentially earning it with a great answer.
I absolutely agree with this sentiment. However, when it comes down to the words used in the dialogue box, there is no such contract. The contract I actually "signed" was to "Reward existing answer". If the system intends that I "reward an existing answer unless a better one is posted", it ought to say that.
Perhaps Reward existing answer ought to be removed as a bounty reason.