No one is going to compose an RFP response, research report or a marketing analysis using weblog software. Personally, I find it challenging to write a 250 word blog post - much more than that and I switch to another tool and paste it in.
In the real world, documents are authored in a word processing application and stored in a document repository (local disk, share drive, or document management system). What you find in the knowledge blog (k-blog) is a description of the document along with a link. Link analysis could then be used to create metadata describing the linked document.
I would love to see k-blogs replace email for standard project communications. But I don’t see corporations implementing that change anytime soon. We could copy email contents into the k-blog, but it might be preferable to link to the underlying email just as we link to documents within the information repository. That would preserve the relationship between the email in a thread.
For a knowledge management solution, we need integrated search that spans the k-blog, document repository and email archive. If there is a true DMS as document repository, then it makes sense to use the DMS as the underlying content store and as the search engine. Otherwise, it makes sense to handle it as an internet search with spidering of the document repository.
BTW, it may be legal to search the corporate email archives. But I don’t think that anyone wants all their email searchable by colleagues. At the very least, search should be limited to publicly exposed email threads.