I get the sense that between 1.2 1.3 and 1.4 this isn't a big issue. Some of the added features dont get rendered since nothing is triggering checks or filling in data for those variables. But are there any issues with 1.4 themes on 1.2 for example?
I worry about it a little more with 1.5 now that the 2 official plugins have a version split. It used to be a little easier to just support the latest version of the plugin but now if you are supporting 1.4 then you are also supporting the latest of the older versions of the official plugins.