This is something that REALLY annoys me. A thread can change direction within a couple of posts - the OP gives more information, changes their opinion etc - yet still, posters give their opinion based solely on the OP, taking no notice whatsoever of the rest of the thread.
It comes across as horribly crass sometimes too.
I do appreciate that you don't always have time to read the whole thing, especially if it's pages long - but then, save your opinion until you DO have time, yes? Please?
Grrrr.