Going against the grain, maybe, but I don't mind trolls all that much. Generally they, like the stillbirth/induction/tokophobia one, raise real issues that can bear discussion. I've seen many threads here deleted because the OP was trolling, which I thought should have been left standing. They contained genuinely helpful & interesting replies, and those could have helped another person who'd arrived here by googling a similar issue.
Problems arise when a troll is unduly aggressive (so will be ignored and/or deleted anyway) and when respondents get very involved with the thread. The American cancer troll had people giving up time and money to help her, even cutting their work hours to keep her company via webcam. The DSM is currently investigating cases like this, to see whether it should be considered a manifestation of Munchausen's Syndrome.
Certainly, some posters are deeply distressed or confused and that can make them come across as trolls (I'm thinking of Boxroom, there are others.)
I've seen Mumsnetters go out to help others in real life; I've always assumed they'd 'known' each other for a while. If that happened for a poster who turned out to be a troll, I'd be gutted. Basically, it's important to remember you do not know all your readers - especially on a big site like this one. Only 1 in 10 forum readers post, so you're always talking to strangers. I do think troill threads can be useful even if the OP was insincere, though.