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I understand that a comment should add (or ask for) information, therefore a "thank you"-only comment will waste people's time.
While it is not encouraged to give thanks only on Stack Exchange, I still think that it is my (at least) responsibility to thank for a help. Plus, that extra comment will tell the person who give me value information that I get their message, and it is truly helpful.
If I am them, I would like to have a feedback to know whether my help really helps. After all, if I feel wasting time to read one more comment, I wouldn't spend time helping at the first place (but this is highly opinion, I admit). I think other people will agree with me too.
My point is: Even a simple thank you comment transfers the bit that the helper's help actually helps. I say the helper in general, not explicit the answerer or commenter because:
- Yes, simply accepting the answer will give them 25 reputation points as thank. But there may be more than an answer that actually helps you, and while you can't accept it, that answer do give you a new way to think about the problem. I wish I could give them 25 reputation points without the need of accepting.
- Sometimes someone just drop me a keyword, so that I can google better. And in general, an upvote can not give the commenter reputation points, so how can I inform them that their information is helpful?
When reading the question Is it acceptable to write a thank you in a comment?, I think a thank you comment lays between give constructive criticism and add relevant minor information.
So, what do you think?