The Goals and Benefits doc is the canonical document addressing this question:http://puremvc.org/component/option,com_wrapper/Itemid,31/In a nutshell, the basic value-adds over a home-brewed MVC is this:
It's documented, unit-tested, bug-fixed, production-quality code and there are plenty of people in the world who use it. (Just join the PureMVC Developers group on Linked-In to contact professional PureMVC developers worldwide: http://www.linkedin.com/groupInvitation?gid=71061
That means when you're in over your head with version 6 of your world-dominating app, you'll be able to find people all over the world who understand not only the basic principles upon which it is organized (MVC), but also the implementation of those principles in the real world (PureMVC).
Additionally, these classes and their roles responsibilities and collaborations have been ported to plenty of other languages, so you can re-apply your learning on other platforms, and port your code to other platforms in a more straightforward way than burn-down/rebuild.
And finally, you carry forward with you an unchanging methodology for handing this unchanging principle of code organization. You can look at code you wrote years ago in PureMVC and have zero issues understanding the organization of things.
Personally, since I released PureMVC in late 2006, the few times I've coded without it, I've felt adrift. Not as though I couldn't solve problems without it (I did fine for a good 20 years up to it), just belabored by having to solve problems with a different mindset and set of tools. And often the tools had changed since I last used them. PureMVC was fully-scoped before it was released. It was bug-fixed to maturity, and left at that.
So you're not going to continue having to upgrade your PureMVC library and figure out how we've decided to do things now. Out of all the tools you're forced to do that with, PureMVC wants to help you by leaving a good thing be. We extend it with ala-cart utilities rather than new versions of the framework that may break or force you to change your old code or how you write new apps.