When discussing about an answer before posting it, commenting is essential. We can add spoilers in questions and answers
but not in comments. Anyone who reads the comments can see the spoilers. Why can't we add spoiler tags to comments?
This is a bit of a complicated matter to address. If you can provide a specific use case where the use of spoilers would be beneficial in the long term, such a request could be better prioritized.
While the subject of spoilers, their nature, how to avoid them, and how to use them has been the subject of numerous feuds debates, one thing is for certain:
Each and everyone of us, as individuals, has different opinion and tolerance to spoilers.
I myself enjoy them. My reasoning is that, if I know how something ends/happens, I don't need to worry about the conclusion and can focus more on the events in between. I know many people would disagree, but the act of the matter is that dealing with spoilers is more so a matter of netiquette than necessity.
No one can really agree on how long a spoiler stays a spoiler, nor can we force them to agree. Common netiquette dictates that unless the OP specifies otherwise, spoilers aren't welcome. Spoilers tag are more of a band-aid of sorts to mediate the situation.
If you have a tolerance to spoilers so little that you don't want to know anything about the future plot points, you'll have little choice but to avoid the entire Internet and other people until you're up-to-date.
On the surface, someone concerned about spoilers simply has to view/read new episode/chapter/volume the moment they're available. The act of the matter is that we live on a global scale, thanks in part to the Internet. Due to the circumstances and bottlenecks (personal or otherwise), we can't all enjoy everything simultaneously. With anime and manga, Japan as a distinct and lasting advantage, due to factors out of the control for most people, such as localization and distribution. With that in mind, spoilers are an inevitable part of the everyday -- if you're a year behind on a series, it is almost unavoidable that you'll encounter at least minor spoilers for anything popular.
As a Q&A site, spoilers are unavoidable if one desires a complete answer. But realize that not everyone everywhere has access to the same channels. On the various StackExchange sites, there are multitudes of users of different experience levels, from countries of varying social climates and statuses. Some might be better off than others, while others might not. It's best to take netiquette into account when posting about spoilers.
Sure spoiler tags are nice to have, both in the body and in the comments. Are they absolutely necessary? I don't think so.
There are times where it's alright to post spoilers. For an example, if the title implies (w/o spoilers, or saying "(spoiler) in the title"), that there will be spoilers (how does X obtain Y)
Just as you should avoid spoilers in the question title, by reasons of netiquette to fellow users, you should avoid writing so on the comments as well.
Why not the in the comments you ask? I feel that mentioning spoilers could facilitate unnecessary discussion chains. Unlike sites like Reddit and YouTube, whose content and usage revolve largely around the body of content and peoples reactions, we're less so a forum, so extended discussions via the comment are a bit counter-productive. We have our chatroom for that. If you have something significant you want to point out (i.e. details are off or out of context), post an answer addressing just that. If you want to have further discussion on the subject, use chat.
In conclusion, I believe that having spoiler tag in the comments is not very conducive to the purpose of our site. We are after all not a forum, but a Q&A site. We ask questions to get answers. Everything has its place, discussions about our site policy is be done in our meta (or Meta.SE for StackExchange commnity-wide policies). If an answer is wrong or inaccurate, you can inform the answerer of the discrepancy and downvote the answer to let others know, but at the same time, provide a source of reference to back up you point.