We usually highlight spoiler topics so anyone daft enough to go into, say the "Opinion on Endings" topic deserves what thy get for being stupid.
I guess its useful for topic where we're not talking about storylines and someone puts it in, but... My crib with spoiler tags on every forum is that if one REALLY doesn't want to be spoilt they shouldn't be on a forum, because members WILL talk about the latest bit of the series. So expect spoilers. If your going to have a spoiler tag, you need spoiler rules to enforce the use usually, otherwise it doesn't always work unless someone is actaully decent enough.
As for my exeperience for Spoilers... I edit a One Piece wikia, I visit a OP forum... Spoilers for a series is impossible to avoid. I've tried it 5 times in the last 2 years and only managed to keep the reaction fresh twice. And on the forum the OP, yes we have a spoiler tag but we also have spoiler rules and its still getting released into the main stream forum.
So yeah, my point is spoiler tags are NICE but they are really going to be used by the "Decent" guys.