Once you go MVC, you never go back. Properly using an MVC framework, particularly with web-based programming, completely alters you ability to look at your old programs the same way again.
Now that I’ve graduated to a world where new development takes place in Java/Struts or Ruby on Rails, going back to the old Perl or PHP program is a nightmare.
This is not just because out of the box Perl and PHP don’t provide any structure to web programming. It is also because doing things in a very structured way changes how you think about program flow.
Needless to say, everyone here at thoughtbot, myself included, has to continually stop ourselves (and each other) from rewriting each of our old programs from scratch every time we go back to do some maintenance.