From what I've seen for themes, the idea of manually calling a custom banner like the Regulus theme is a new idea for WordPress. Most themes were created with the idea that one could easily modify the theme but with the WordPress MultiUser system that we run here, themes are shared from one sourse across the servers and that's a new concept. I have seen theme designers begin to go back and start to modify their older themes to allow this. There's also a plug in for theme designers to help them do this.
It'll take time I'm sure but I have a feeling that more themes will begin to allow this in the future.