I use spoilers to hide information that might not concern other people exactly so they don't waste their time reading whatever I put inside the tags.
I think people could mistake a spoilered post as being unimportant so they skip over it, just to find out later they missed something they would of enjoyed reading because it was hidden behind a spoiler.
I'm not sure how I feel about requiring long posts to be spoilered, I think I would lean more towards not requiring it though just because people could mistakenly skip over it while scrolling through pages.
Just my thoughts on the topic