They're good for adding more information but bad for establishing notability (as per Wiki policy) because their reliability is contentious.

So yes, there are reliable blogs, but a post from Scoble and no mainstream media hits won't save an article from the deletionists.

I don't get Wikipedia at times. How is this article notable?

http://en.wikipedia.org/wiki/Firearms_in_Miami_Vice

(I have use the wikipedia random page as my homepage)

But no commonly accepted way to validate their reliability

As if newspapers and books are reliable. I'd trust a comment at this website about YC by a YC applicant over a Business Week article.