reprint - 5:55 pm on Apr 17, 2008 (gmt 0)

Ok, ergophobe, following you now i think

"If I post the same content to ergophobeBlog.com, ergophobe.blogspot.com, ergophobe.typepad.com and ergophobe.blogger.com, I have now split my commenting community in four. "

In this case, yes you are splitting your community.
I think the more appropriate case for posting to multiple blogs is where there is overlap.
For example if you have a blog for family, blog for friends, blog for work.
You have a baby, you may want to post the same announcement to all three blogs.
Your aunt gets married, you might want to post to the family blog and friends blog but not the work blog.
I agree that i do not see the point in having 4 completely identical blogs unless people are browsing blogs across a platform and discovering blogs that way rather than following links from blog to blog or using a search engine.