Firstly if a comment contains a helpful addition to an answer (which does not fundamentally change the answer) then it is good to edit that into the answer. If a comment contains a nearly helpful addition to an answer, but requires a tweak then tweak it and add the result to the answer (leaving the original comment as is).
Secondly (and this is my real answer to the suggestion):
People are going to disagree, that's why both questions and answers receive both up and down votes. That's why we have a voting system that allows people to disagree and do so in a fair way. There is a need for people to express why they disagree or in some cases, clarify why they agree or bounce suggestions off the community before making an informed answer or edit. Comments allow people to do this in a way that does not allow others to confuse or destroy the thing they've said.
I believe that if comments were editable then people would try to edit them in order to stamp out the comment's content just because they disagreed with it.
To put my view in very simple terms:
- Questions / Answers represent the community voice as a collective.
- Comments express one individuals voice (free from community interference)
If you allow people edit others' posts then there would be no way for an individual to have their own voice.
It is my view, that allowing an individual to express their own opinion as being separate from the community is a good thing which allows people to make more informed decisions when reading the content. It's necessary to understand why some people disagree with an answer that has been accepted by the majority to determine if that disagreement is something which applies to your situation.
Edit: (And finally) Voting
I find it interesting that in any voting system, people argue about the meaning of a vote. In politics (democracy) some argue that a vote is nothing more or less a statement of who they'd like to rule the country. But that view ignores the tactical voters who vote "to stop the other guy getting in". Tactical voters vote for someone who they don't want to rule the country.
In stack exchange this is even more complex. A down vote can mean so many different things. Of course there is an official definition but it is incorrect to assume that everyone follows that. The community effectively decides for itself (each individual for them self) if something deserves an up or a down vote based on each individuals interpretation of what that vote means.
As an example, I saw a very well worded and simple question heavily down voted because a cultural misunderstanding (most likely) led people to believe the question was in poor taste. The question was "How do you find someone's bloodtype through the FaceBook API". The cultural misunderstanding was that in some cultures blood type is viewed like "star sign".
This odd voting can't be corrected, harmonized or moderated in anyway because that would descend into the moderator's opinion ruling.
Comments - unadulterated and free from interference - add color to an otherwise monochromatic voting system.... And thus I also disagree that comments should only be temporary. Some comments deserve to stay as comments.