What wants to integrate with what? Many who have a new app or category of app decide that the way to drive adoption is to integrate it with email. Same for blog aggregators.
While I am not convinced that this is the right way to go, I don't want to contest that approach necessarily.
I want to raise the thought that not all apps that seem similar do better when integrated together, and conversely, some apps that do better together actually are not similar at all. In other words, similarity is not the test for integration.
My exhibit #1: Why not integrate email and instant messaging into one client? Seems like a very obvious idea, and yet it's not been done (effectively right) and in my opinion it's a bad idea.
Superficially, IM is just very fast email. Why not have a checkbox on your mail client that says: "send instantly."
But no, in my opinion I don't think that would ever work because the mode of use of IM is totally different from email. And the purpose in the user's mind. IM is lightweight and transient, email is richer and more permanent. IM is small and casual, email is bigger and more formal. If we decided to add a 'sent' folder, cc: lists, and rich text editing to IM, we would break it.
I don't know that I believe as firmly that the same is true about email and blog aggregators, but it's food for thought.