<p>One of the stated goals of the PureMVC framework is to avoid platform dependencies in order to be portable. Considering that because of language and API differences application code will always be heavily dependent on the platform, and that avoiding platform dependencies makes the framework reinvent the wheel and/or only provide a least-common-denominator feature set, in what way does the portability of the framework benefit me as an application developer?</p>

I've worked with PureMVC. They're trying to implement their stuff in quite a lot of languages. You may be right about the least common denominator, but overall, it's not a bad framework, and I've seen a really nice AS3 app in PureMVC.
I don't think they're talking about portability in terms of porting actual code. The idea there is more that you're using a generalized MVC architecture, which you could apply to other projects and other languages.
They're trying to say that if you become familiar with the PureMVC pattern, you could potentially come into a new PureMVC codebase, even if it's another language, and you would already know the lay of the land.
You might also say that developers who develop good PureMVC skills are likely to develop good habits which will translate as they go from language to language.  But then again, maybe not.. for the reasons you mentioned.