I'm probably an edge case user but I don't save stuff to Pocket because I want summaries of stories. I save them because I want access to the actual content.
For example, I do save articles I can just read. For that, a summary service would be great BUT that is true with or without Pocket. On the flipside, if I saw stuff like a blog post on technical code someone wrote about for something, more than likely I want to save that reference for later (true for a huge chunk of stuff I save or the type of things I save).
That said, I came across several people who have either worked on summarizers or talked about it but nothing seemed to have ever took off on those. I can't speak for others but in general I prefer to read content for myself.
I've made a naive summarizer that seems to get the job done for the summaries part: http://breue.com/summarizer
I'm just not sure what the final product would look like or if there would be enough of a reason for people to prefer it over Pocket.