I agree!
I do watch Movie trailers but normally only the first one as after that they become too spoilery. I especially can't stand previews for upcoming TV show episodes, I mean you already know if you are gonna watch the show or not so why go into it knowing part of the resolution to the just finished ep you watched?
I personally wish TV OT threads would all just be discussion up to the latest aired episode only and ideas of what's going to be happening, instead of the black bar filled threads we end up with now after an episode has aired and people are desperate to discuss the previews. I mean once the show has aired you should feel safe going into the thread but you never know anymore with how terrible some people can be with their spoiler tags.
It's a bit of a sticky situation really but I agree with you for the most part. The only preview that I think should be discussed in a TV thread (with spoilers) are the post-episode "here's what's happeneing next week" previews. Extended previews that aren't aired after an episode, interviews with cast and crew, and leaked info/rumors should all be, at the very least, spoilered.
Not everyone wants to speculate to that level and it always sucks to have stuff spoiled. Just by glancing through this page, I'm pretty sure I've been spoiled by not-so-vague discussion of what's been said behind spoiler tags.