Thanks for being so honest:
So, after reading your reply I wasn't 100% sure how to justify this, but by the end of this reply at least I've talked myself into it pretty well ;)
I do share your interest in cooperating small applications. Infrae's Railroad is quite a bit like it; provide a WebDAV repository that multiple front-end CMSes can share, while it turns around to ask the CMSes (through HTTP) whether someone is actually allowed to do an upload or download. Tramline is also a bit like it; it's application functionality that sits within Apache and handles things on the request level, but gives functionality to backends which might be written with any framework or language.
Since you're advocating it so persistently I figured you'd have more wisdom on where one makes the decision to use an independent application versus a library. Language independence, something you mentioned, may give rise to more widespread and long-term reuse of code. This is quite interesting. You pay for a deeper investment than with a library, but the gain may, potentially, be a bigger community of users. It's a bit like the benefits one can get from using standards, except that the investment is far less than in creating a new standard altogether.
Obviously you also write reusable libraries/frameworks; something like SQLObject for instance I can't imagine as a standalone web application. Perhaps you can. :) The question when one would make such a decision to go for a separate application is an interesting one. Making the answer more explicit sounds valuable, so we'll see where the thinking brings us.