I have two major issues with this:
Lousy answers tend to make for lousy comments. Not always of course; the classic examples are answers that simply ask the asker for more information, though even these stop being useful after that information has been provided. You should generally just delete these; in some cases, it may be worth breaking out of the queue and either flagging for a moderator, or just posting the comment yourself.
I've seen waaaay too many suggestions for "convert to comment" as a tool for denying reputation to the authors of answers that are actually answers but don't meet someone's quality standards. Link-only and code-only answers are the classic examples here. This is really a cop-out: if you honestly believe the answer is utterly worthless, foul garbage cluttering the site, then vote to delete it; otherwise, break out of the LQ queue and down-vote it.
Of course, there are legitimate cases of answers being posted that are simply better off as comments: answers that are replies to other answers, exist only to provide auxiliary links, etc. I don't think it's wise to consider "convert to comment" a panacea though: the proper action may actually be fleshing out the answer as it sits or editing the information into a different answer. Forcing you to break out of the queue to handle these exceptions recognizes their exceptional nature, which is also reinforced by the fact that only moderators have access to a "convert to comment" feature in the first place!
Regarding your examples:
Great examples here
This isn't an answer, but it isn't a particularly useful comment either (though it has now been converted to one). This falls into the category of "I don't want to bother trying to understand your problem so I'll just hand you the documentation" responses, and IMHO these should nearly always be deleted outright.
The best way to do this is to run the server on a thread
This definitely isn't a comment. It isn't much of an answer either, but this was the asker's summary of her solution (and like many such answers, it left out a big chunk of information that was - after the fact - obvious to her). She later explained why in the comments. I edited her explanation into the answer.
Have you tried
Still not a comment. If you re-wrote this as "Use
ExternalContext.getRequestParameterMap()", it would clearly be an answer.
In short, I wouldn't recommend converting any of these into comments. The first one is better off just gone, the last two are fine where they sit.