Sometimes a comment can add great value to an answer once its content has been incorporated into the answer. Perhaps correcting an error to an otherwise good answer, plugging a logic hole, rendering a more elegant solution, making an important clarification to explanations, etc.
Once the answer has incorporated the comment/suggestion, it would be nice to credit the commenter with a "slice of the pie" - ie share reputation gains between the answerer and the commenter, as recognition of their contribution as "co author" to the final answer.
The simplest way would be to make upvotes on comments yield reputation, but that barrow has been pushed before.
A few possibilities I could think of for sharing rep are:
- The commenter can be added to the answer as a "reputation sharer" - reputation gains from the answer are split 50/50 from then on
- The answerer can make a "donation" of reputation from the answer to the commenter - eg if the answer has 4 votes, they can donate up to 40 reputation (there may have to be a requirement that this donation comes off the rep cap, so answerers don't distribute their excess rep when exceeding their rep cap)
- Upvotes made by the answerer to comments on the answer do get reputation (maybe 5 - transferred as per previous point)
Why bounty doesn't quite fit:
The bounty process is just too long winded and doesn't have an on-going effect if the answer turns out to be a really good one. The bounty is too delayed. They didn't answer, they just added value - I just want to give the comenter a pat on the back right there and then, and I don't mind giving up some erep to do it.