MVC is a wonderful framework to work with. Unfortunately, there is lots of existing material and sites that are written using WebForms and these require maintenance. Going by the maxim "never leave the code untouched", I reckon that these sites and code shoud be refactored bit by bit and transitioned towards using MVC.
It is, of course, not practical nor beneficial to rewrite whole applications but adding new functionality and refactoring the old should also encompass replacing bits and pieces with MVC framework. So, in that regard, here are some pointers as to how to mix multiple existing technologies, all hosted on IIS: