I don't think a theme framework in core is ideal. What's packaged with WordPress should be starting points for further development.

For plugins, we have Hello Dolly. For themes, we have Default and Classic.

These are meant to be very simple extensions that allow developers to learn. If we package a powerful theme framework in there, it could make some brand new developers run for the hills. I probably wouldn't have gotten into theme development for WordPress if the Default theme was a framework like the ones we see today.

We also need to remember that any theme is a framework. Essentially, we already have two theme frameworks built in.

I'd rather see a more solidly-coded Default theme built in. I absolutely hate the way it's coded now. The focus should be on best practices, but it should also be simple and understandable enough for newbies to learn from.