Sometimes they can be helpful!
Yes trolls can and do post and its annoying but being able to discuss it afterwards can help people spot them in future and maybe not be taken in. Sometimes a troll thread can actually bring up interesting subjects that should be discussed, I recall there was one about financial abuse (cant remember if that turned out to be a troll or not). That should be discussed! Some women live with the most appalling situations with no access to money, and the level of disbelief on that thread shows how little people understand.
Also one thread about a troll thread is easier to keep an eye on than many people posting several threads about it as they may have missed the original deletion. Seems a bit silly to keep doing that when leaving one "What happened to X thread?" running keeps everyone informed.