If this were not the case blog posts would not be able to get high rankings. ie.- mydomain.org/myblogkeyword/mypost.- Doing it this way also means that I can diversify and still be adding more content to my website making google happy.
I have set up several blogs using this system and manage to get them on google page one, sometimes in a matter of days ( I recently made a post that appeared at No1 instantly against close to half a million results) and I don't have to worry about submitting the sites as they get indexed automatically within minutes.
So why should I buy a new domain for each micro blog I set up?
By the way to make matters worse (in theory) I use the wordress multiple blog system so my blogs are actually - mydomain.org/mainblog/subblogkeyword and I still get them onto page 1