Well...I assume the ePub 3 will work...but only on devices/apps that support ePub3. The problem is that there are very few devices/apps that support it...right now. Developers ARE working on it, but we are definitely in the beginning stages of adoption. I wouldn't say they are trying to "rip you off" unless they are trying to tell you that it will work on "all devices".
It is possible to create an ePub 2+3, which is a hybrid that can be used on any device. Devices that do not support the advanced features of ePub3 (video, audio, etc) would only show the ePub2. Devices that do support ePub3 would display all of the advanced features.
[disclaimer] I have not created or worked with ePub 2-3, but you can see examples of them at the
Azardi website.