They're often detrimental, in fact <.<
I never watch previews anymore unless it's like, something I already know what will happen in (like, say, Game of Thrones, since I've read the books).
There are some good previews that trick you into thinking one thing and then BAM plot twist (IIRC, Supernatural did this a few times...), but they're few and far between and generally not worth sitting through the spoilerific/bad ones for.
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