I think the problem as I say it's that is a general manga thread. If this was a Naruto thread for Naruto readers, it would make a bit more sense to allow spoilers, as they are going to be discussing all day about Naruto's plot and twists and character and whatever.
But in a general thread, you enter and you don't know what people are going to be talking about.
A seinen manga? shonen? shojo? josei?
-Something old, but that you wanted to read someday? What do do you then, risk to read because you want impressions to know if it's really worth reading or not, or don't risk reading because maybe there are big spoilers?
-Something weekly and new?
-Something that just released 14 minutes ago and you didn't even know it was released? Well, if you aren't careful enough and have some bad luck, maybe you read a spoiler before noticing what was happening.
I'm not saying we need strict spoil rules, I don't even use spoiler tags for lots of stuff. There is a difference between small spoilers and big spoilers. Difference between something just released one hour ago and something released 2 weeks ago. Difference between talking about the plot in a slightly general, abstract way and talking about the plot writing exactly the events and all the spoily stuff.
Usually you just need some common sense, education and context.
Well it's in the OP so I'd consider it a rule for the thread
Yeah we know. Now we are discussing how good (or bad) is that rule.