I post my stuff all over the web, I don't want to repost all that to my own personal site. Posting to all the other big players is great for discovery, grabbing all that content to post to my own site is great for email footers, business cards etc I built http://qiip.me exactly for this, see mine at http://qiip.me/edlea
That's not a pattern I've noticed generally. A lot of the sites I deal with are very specialised with content.
For example, posting a listing to Etsy.com requires some very specific details which are not relevant to other sites. To post an Etsy listing, share the same photos on Flickr, share that content to a blog post would require a lot of data at the central point to spread to all those services.
I think it's easier in the cases I've seen to post to the end services and pull that data back in.
Shared fields are not often more than Title, Images, some Body Text. Then there are hundreds of smaller options from price to tags to materials to photosets to choose from.
The other issue here is that where cross posting is easy, usually the main service already provides it, i.e. posting from Instagram to Flickr. If you include services like http://IFTTT.com then you're pretty covered for this case.
Who would want their personal site to be connected to so many other sites, all whom are more powerful and make more money on your site than you?