1 they are very annoying. You contribute in good faith and then you get the big reveal.
2 they do not accurately reflect the situation because they are written with a bias designed to get you to give the desired response
3 the poster creating the reverse cannot know what is in another person's head so they attribute thoughts, feelings and motivations that are their assumptions, and not necessarily an accurate reflection of the other party's views
4 they are manipulative. It feels like you are being played in order to make the poster feel like they've 'won' a situation.
I realise it's not a biggest problem on here by a long shot, considering we are slap bang in the middle of the biggest troll season we're ever had, and you at MNHQ must be feeling like a bunch of Elmers going after Bugs, and I know that you never know the truth of any thread, reverse or not, but it's bloody irritating when you get the ta-dah, actually I am the other person... and I just thought I'd ask what your position is on reverse threads. If you don't give a shit, that's fine, I'll shut up whining about it. 