Nobody seems to be thinking comprehensively about this.

a) From the poster 's perspective: Technorati has been around awhile. The poster can scan comments on other people's blogs as easily as on his page.

b) From the commenter 's perspective: Commenting on a blog feels different than posting on one's own.

c) From the reader's perspective: Technorati isn't that useful. Reading a list of comments on a blog is much easier than searching technorati. Nobody bothers.

d) From the maintainer's perspective: It is natural to feel possessive about your blog, and to try to keep it clean, and to feel anxiety about not being able to stay on top of maintenance. Doing this is a burden.

e) From the commenter/maintainer's perspective: it's too much work to start your own blog just to respond to others. Especially if you care about the persona you build, creating a blog gets into designing it, wondering where to host it, etc., etc.

d and e are arguably current limitations of technology. The rest are not. Where you write will hugely influence who you might end up having a conversation with.

So if you want to build a community, or to be in the business of letting your readers talk to each other about what you write, enable comments. If not, or if you suffer from OCD, move along. There's lots of others building communities where they can chat about you.