Great. My responses is the same. If you have a foundation that can handle users/visitors/customers, roles, security, url rewriting, theming and multilingual duties (to name only a few) and it supplies a platform for your custom code as a module, why would you want to reinvent it? If you have a platform to build on, you can concentrate entirely on the app itself.
NOTE: I apologize if this is looking like an advertisement for Drupal. I don't mean it to, but it is the CMS platform I know best but the idea can be applied to any robust CMS with a well documented API.
A couple of years ago our team built a tool that engineers use to model fluid dynamics when they design water systems for everything from housing developments to entire cities. We built it in OOP/MVC and then wrapped it in a Drupal module so that it would integrate with a Drupal intranet we also built. This method allowed us to dedicate our time to designing and building the application and Drupal took care of everything else; page generation, navigation, security, user management etc...
You'll do what you feel is right for your project obviously but I think it speaks to the flexibility of using a good CMS platform when you look at what others are using for sophisticated sites and apps. I find it telling that Java.net, Sun, Adobe, Linux Foundation, Google and even Digg are using Drupal when they obviously have much more technological resources than you and I:
Good luck with your project.