As would I, as "pushing the data onto the client and using it from there" is by far my biggest issue with Silverlight. I was surprised to see a framework that specialized in doing this.
I wouldn't say sproutcore specializes in pushing data to the client, it really is a 'full stack' client side framework. That said, this is one thing that sproutcore does do quite well, the interface for hooking up to JSON datasources is very simple and effective (particularly useful are cascading data sources, which they've recent added and really cleans up the code where there used to be huge if-else if blocks on queries inside your data source file).
What don't you like about that approach?
There are a lot of things I don't like about it. But in all honesty my dislike might actually be how RIA Services implemented the approach, not the approach itself. I am currently writing a big blog entry going in to details on this. I'll let you know when it's posted.