When I first took up blogging in October 2005 I had already learned, from extensive reading around, that posting a comment without having first read right through the thread was pretty much universally regarded, at least by those people whose opinion one might reasonably respect, as naff and non-U.
There were obvious good reasons for this. The great virtue of the comments threads is the way they develop a narrative of debate; the way they work through the various possible positions and objections; the way people build on and develop each others' ideas and arguments. (And jokes.)
It might just be me, and it might just be the blogs I read -- but lately, over the last month or so, I have noticed a considerable increase in the number of people who jump in when they have clearly not read through the thread. They ask questions that have been answered two days back; they put forth cases that other people have already made; they cite sources that were discredited on the thread a week ago.
(I should make it clear before I go on that I'm mainly talking about the big group blogs.)
When called on it, these people tend to say loftily 'I was commenting on the original post' or 'I haven't got time to read the whole thread!' (Implication: 'All these comments are crap, except of course for mine.') I'm deliberately not putting illustrative links here because I don't want to cause needless irritation, vendetta responses, blind bloggy hatred and so on. All sorts of different people have started to say/do this kind of thing.
So many blogs, so little time, and indeed there often is a longer thread than one has the time to wade through. Fine: don't comment, then. Surely if one has not read the thread then one is in no position to be commenting, because actually it's not possible to comment effectively on 'the original post' if you're doing it in splendid isolation. It seems to me that the ever-changing Gestalt of a blog entry is not just the post itself but the sum of its commenty parts, and to jump in and comment without reading the thread is to disrupt the flow of argument, bore and annoy everyone who has been following the thread properly, and make a dill of yourself into the bargain.
I've never really bought the argument that the comments and discussions are the main thing that makes blogs wonderful. It depends entirely on the blog, and any comments thread full of trolls and barking nutters really is a complete waste of your reading time. And now, with comments threads being tangled up by people who aren't following the discussion, there's even more reason not to read all the comments threads automatically.
So I'm in a lot of sympathy with other people who don't read them either. The difference is that, not having read them, I wouldn't dream of chucking my two cents' worth into a discussion that I haven't followed from the beginning.