They're not going away any time soon, but by their very nature, Sturgeon's Law applies twice over.
Of all weblogs of any stripe, 90% of them are crud.
Of the ones that aren't, 90% of the content on them is crud.
They do offer places for people to post their opinions, draw conclusions, interact with others, and share information. The instant-interactive factor is pretty nifty, when you have a site owner that actually does interact with their audience.
But the whole media rattletrap about "journalism blogs" and "warblogs" and the like is hyped bull. They're a buncha newbies spotting a pattern and getting all thrilled because they found a way to pull order out of chaos They don't seem to realise how small the trends are, or that they've been spotted and dismissed half a dozen times already.