Just as Facebook and Twitter are built into the Windows Phone OS through the settings/accounts and People Hub system but still have a full third party client to access the service, so does the Chinese network.
There are three major changes in the update
- Share and see more pictures
- Add expressions
- Upgrade Weibo publisher
With Windows Phone's push into the Chinese market, working with the popular services in the country is vital. Microsoft's mobile OS is built around the idea of tight integration, and when there are over half a billion users of a service, you need to be supporting that service both in the OS UI, but also (presumably) ensuring their native app is promoted and easily available.
Sina Weibo users can pick up the update via the AAWP App Directory.