Its not clear cut what all is included with semver. Is switching from a C89 to a C99 a semver-breaking change? Is using a Python 3.1-specific function a semver-breaking change? Is fixing a bug or adding a new feature a semver breaking change [1]?

There is no clear cut answer in all of these situations and applying purity tests to semver instead just push another crowd of developers to abandon semver completely, making it hard to get any information communicated from your dependencies up to you.

[1] https://xkcd.com/1172/