Rooted devices and custom ROMs are slightly different things. If you take your stock ROM and just root it, you will change the privileges, but you won't change (almost) any of the system stuff. These rooted devices we support. If you install a custom ROM, you (usually) root your device and install a custom ROM which means certain parts of the system are different. These devices we will probably never officially support since everyone can build their own custom ROM (since Android sources are publicly available) and it is not possible to support all of them.
Filip