Frame in a foamie with Azek?

So what if I were to use Azek to frame in my foamie. You know, for the door frame, the window frame, the spars, etc...
It doesn't seem to be that heavy. And it certainly won't rot. So what are the downsides?

Mike...
It doesn't seem to be that heavy. And it certainly won't rot. So what are the downsides?

Mike...