In answer to the question "why use Python?" the answer is so that we can leverage the power of Python, the Python Standard Libraries, the ability to wrap C code, work equally well across a large number of platforms, etc. PythonCard does not use a subset of Python or try and recreate HyperTalk.
I was a HyperCard developer for ten years and loved HyperTalk, but I've moved on. I wasn't aware of Revolution, it looks nice, and I hope they do well.
However, I think Python is simply a much better language than HyperTalk ever was and Python has the best user community I've ever encountered for a language. Frankly, at this point in my life, I'm through with using languages controlled by one company. Even Java suffers at the hands of Sun.
It doesn't hurt that Python and PythonCard are free. I'm not saying people shouldn't continue to use a tool that can run or import HyperCard or uses a variant of the HyperTalk language. PythonCard won't be importing HyperCard stacks anytime soon. Freedom of choice is a good thing, so use what suits you. What we hope to do with PythonCard is take the good parts of HyperCard and what we've learned in the 14 years since HyperCard came out and build something much better that we can use for the next ten years or longer.