After an update is released I go roughly through it while doing regular browsing, and if I don't see issues I push the compatibility - however, keeping track with pre-release versions and doing real test cases in separate profiles (to not risk loosing any data) takes more time - time I don't have currently. If you want to improve the situation, feel free to test it in beta and alpha releases by yourself, and report if it's working and if not what's not working (via Mail). If I know all issues before the release of a version, I have a good chance in fixing issues and updating the compatibility before the actual release.
If you're like "woah, that would be actual work, why doesn't he just test it early - I don't even get paid for that" then remember, I don't get paid for this either (unless you want to pay for it?). I basically share what I do for myself and this project must fit into my tight schedule (open source developers have a real life, too).
I agree that the update situation on my themes is far from perfect, and I can also agree on the content of your rating; however I dislike you're not seeing any other option than me testing multiple releases.
Thanks for loving the theme :P