I agree; a quick tinker with my 40D indicates that a YN600EX-RT II will trigger other RT lights given a couple of caveats:
ETTL and manual modes work but only for lights in groups A, B and C;
Multi I'm not sure works properly; and
Group works but only for manual mode for each group and only for A, B, C although it does allow different power settings.
The YN-E3-RT on legacy mode may be a partial workaround but that's another story. In any case I maintain that few extra bucks for RT triggering is worth it even if your current camera only partially supports it.