Compatibility with what most people use. People who use unofficial HEADs should know what they are doing and particularly be aware that defaults may change from one version to the other---obviously they've had this experience at least once. Those who just run "aptitude upgrade" or equivalents to be fed the latest stable version are less likely to pay attention to these things and just expect their applications to behave the same. You could just warn() people who haven't explicitly set a protocol family about impending changes in the default for a while and then switch it.
