I need to run the latest Python versions, including alphas and betas, though, and have everything just work even if the system and local Python versions change under me. Pinning Python would make it difficult to contribute to CPython, or make sure my libraries work with new Python alpha/beta releases before stable versions are released.

Virtualenvs based on Pyenv's local Python installs on Linux works well, and virtualenvs that were based on old system Python versions still work even when I upgrade the system Python. This is where I'd get linking errors with Homebrew that I don't get with Linux.