It would be simple though to limit how many times a spoiler can be used in a single post ...like say 5 per post max....that keeps spam down a lot.
I have used spoiler myself on my own forums when making very long posts to hide each section so that it makes loading times shorter since not everyone wants to load a post in a topic thats super long. Specially those that still use dial up connections due to broadband not being available in their area.