There's nothing wrong with an answerer deleting his or her answer. Forcing answers to be permanently visible once posted would be silly and would discourage some people from answering.
Normally users will delete their own answers either when they realize that their answer is incorrect, or when they feel that the other answers to the question are better and that their answer adds nothing new to the discussion. This is fine and is in fact expected.
Additionally, it is not expected that answerers will sit on StackOverflow all day long watching their answers like a hawk and responding immediately to any comments you leave. In the question that sth referenced (for which, as a 10k user, I can see the deleted answer) you seem to have gotten angry at one of your answerers for not responding quickly enough to your comments. Remember that answers on StackOverflow are provided by volunteers who are donating some of their time to help you. They do not have an obligation to cater to your every request.