I'm wholeheartedly in the group "for-spoiler-tags".
For me, the harm of "readers needs to move their mouse to see" is strongly outweighed by the harm of "accidently reading/seeing" a solution.
Someone looking for "the" answer needs to do this on a singly (accepted) post only, whereas it is (close-to-)impossible to "not look at" a solution.
Please ignore the next line and read on after the next headline.
Why I think spoilers are good.
It is really hard to willingly ignore facts your eyes are scanning. The problem is worse if it is a picture. Again please ignore the single picture I've placed at the end of this posting. I have deliberately placed it far below, so it should be easy. It might be tricky though, if you want to go through several answers....
Then, there is also another reason why I think spoiler-guarded answers are good. I really enjoy reading good solutions to puzzles I couldn't solve. Sometimes, the answer is nicely split into spoiler-guarded sections like:
The answer is "...".
My initial reasoning was "..."
Hint XY then gave me the idea of "...."
I can go through those one by one and easily decided to take up a puzzle from there. I.e. I read the solution, but want to figure it out myself nevertheless.
A third usage of spoilers - a bit more disputable though - is that several (of my :c) ) riddles were strongly criticised for being ambiguous. I don't want discuss this opinion here, but sometimes narrowing a riddle down too much can really, really spoil it. I think it actually a very good idea to place not initially essential "restriction" facts into spoilers.
Finally, I think spoiler-tags have been created and named to indicate exactly that: Sections that spoil it for the reader. Plain-text (or image!) answers right next to the posed question do exactly that. So if one shouldn't use spoilers there, what should they be good for at all ?? Remove them from the makeup language, if they make the site so horrible....
Why do you think newspapers post the answers to their crosswords on separate pages or at least upside down? It clearly is less convenient for the reader to turn pages (or the newspaper on its head), and they still seem to do it like that for ages.
And with this, I close my contribution to this discussion. I really hope, you don't have a clue what I'm talking about, when I say: Did you notice any elephants in the room?