I don't think there is any such thing as a harmless troll.
Threads encouraging users to share information about embarrassing bodily functions and accidents are quite possibly playing in to the hands of someone who gets aroused by that sort of shit.
When such threads are deleted (poo troll anyone?) many users will feel uncomfortable that they have shared a personal incident in the hope of offering op support only to find that the op is a troll.
And once you've been taken in, you're suspicious of everything. People avoid posting on anything but the most trivial of threads and content takes a nose dive.
I try and give most people the benefit of doubt but always remind myself 'would I be happy posting this if it turned out that it was a troll?'
Troll threads, regardless of how seemingly harmless, ruin the integrity of a forum. It also, of course, gives rise to troll-hunting (and I do mean that in the literal sense, rather than just raising concerns or asking pertinent questions) which is also not great...again this impacts on content as people become reluctant to post anything too interesting due to the risk of being called out as a troll.
There does need to be more of a reaction to reports rather than the feeling that they are just disappearing into the ether - I see MNHQ are trying to address this which is a positive step, they just need to continue with it rather than hoping that it's a phase that will blow over.