My personal opinion is that it is simply good manners to read a thread thoroughly if you are going to add your own comment to it.
It is also extremely frustrating if, as the OP, people 'reply' by asking you something you have already addressed (and especially so if it is something in your OP )
This isn't just an issue with GN by the way. As a veteran of a couple of expat forums, I know it is true across forums.
I tend not to post on many topics, and that is generally because on the whole they do get side-tracked, and usually by a handful of 'regulars' (again, true of all forums, so not making this 'personal' to GN). As soon as I spot the old familiar pattern emerging, I stop reading, as I know the thread will be unravelling and rambling down the 'chatty' route, or all too frequently the 'in-jokes' of a small group who have got to know each other very well and treat most threads as their own personal chat room route.
Chat rooms are a different animal, and I do find it frustrating when forums are treated in that way. My view is that, if someone has started a new post on a particular subject, it is simple courtesy to stay on topic - and to read it properly!
I will just add again that my observations apply to all forums I have followed, so nothing 'personal' to GN