Yes, I would say that writing reusable web applications is harder than writing a library or framework.
I still have to maintain all the old perl code that I wrote before moving to python. I sure wish I'd been developing small apps back then instead of libraries. It would make it much more manageable to convert it little by little to python. In the future, if I decide that I like a different language even more, I can start building new applications in that language and still use the small apps like Tasty without having to completely re-write them.
There's also a decent scalability argument. These seperate, small applications scale out really easily. You can start with everything running on one server and then, if the load gets too high, start moving each mini-application out to its own server with load balancing and caching layers between everything. No additional code has to be written; just some config tweaks.