By the numbers, but out of order:
2) Even when an answer is provided by the acknowledged expert on the subject, it will be there, available for selection, after some time has elapsed and others have been given an opportunity to respond. The same is true for someone who plans to step away -- the answer isn't going anywhere and may still be best when you return.
When I see an answer I think is best, I select it. I don't know when, if ever, I will come back to the question. In hours, the question can be burried and short of making a list, who has the time to review questions read yesterday? (exception for my own posts, I usually wait some time for the post to 'mature')
3) Yes, best answer selections can be changed. Yes, when a member posts an answer that is selected as best by someone, the member receives a notice about the selection, receives G2G points, and may get a warm fuzzy feeling. The part left unsaid is that when a best answer is deselected and replaced, the member who posted that answer is not notified, loses the G2G points unbeknownst to him, and may experience a less pleasant feeling.
4) If I post a G2G question, I claim it is my prerogative to decide what answer provides the best response and the information I'm seeking. I don't care what your stature on this site is, you are not more qualified than me to decide that. And if I decide not to select a best answer, that's my call too. We occasionally see questions in G2G where it's not crystal clear exactly what the poster is asking, and in such a case it's really presumptuous for someone else to decide that his interpretation must be correct, and that his selection of a best answer should trump everyone else's.
I agree with this only for subjective questions.
1) The selection of a best answer to any question has a tendency to discourage the posting of any additional answers or comments, particularly by less experienced members, since it gives the question the appearance of being a done deal. Sometimes those additional answers or comments raise relevant points and add value, even when they are not deemed best.
OK, you've sold me on this point.
If you were to argue for an addition to the text of the help pages to say this, I'd vote for it (I tried my best to write it in Chris Whitten style ):
Consider waiting for some time before awarding a best answer star to a question. Some newer members may see the star and assume that the question has been fully answered and this may inhibit the posting of additional answers and comments. The additional waiting time also gives the original question asker the chance to read and evaluate all of the answers to determine which answer was the best response to their question.
I'd vote for it because it wouldn't be a rule, it would be a suggestion. It would be a suggestion for folks to stop and think about the dynamics of the best answer process and to put some thought into it before selecting and I can't see anything wrong with that.