The points Bud mentioned bother me also greatly, and I've experienced them elsewhere in an identical manner (though that place is now most likely dead). To add to his list, I might mention they make moderating the forum harder in a whole bunch of ways: 1) In the heat of the moment people might feel they can spoil a little since its in spoiler tags, 2) More people will skip reporting spoilers (and other stuff) since they will skip reading spoiler tagged stuff, 3) People might not actually report tagged spoilers even after reading them, instead blaming themselves for revealing the spoiler under the tags, 4) For mods themselves it's an extra mechanical burden to go through a thread clicking all spoiler tags.

Though to be honest this forum is half-dead and spoilers get reported maybe twice a year... So, I guess for me personally the real reason is that I simply learned to hate the tags when reading that other forum, which had no spoiler rules whatsoever, apart from suggesting people use the tags to hide everything they though might spoil something for somebody. Sometimes most posts in a thread were spoiler tagged, and you never knew what was inside them, but reading the forum was impossible without doing it. So, you read an episode discussion thread, clicking the spoiler tag every time somebody had something to say about the current episode (because it would be a spoiler for somebody who hadn't seen the ep yet, but was still reading the fricking episode specific thread, go figure), and then, bam, suddenly what was inside the tenth spoiler tagged post wasn't anymore stuff from the episode at all, but stuff from much further down the game the anime was based on. And that was perfectly okay because it was spoiler tagged. Yeah, I learned to hate the things.