... which is why I'm still a little annoyed because the 600rt cannot be a radio *and* optical master at the same time, unless someone comes up with a clever tech explanation my guess is that Canon removes the command sequence from the (ettl) pre-flash to pressure people into completely switching to the expensive 600rt line.
The technical explanation is not hard, RT offers five groups, optical offers three, that would be eight way communication with test flashes of each group and optical triggering of the three optically triggered groups, between shutter press and shutter release, a total of at least 14 pulses of flash before
the actual flash fire. Pre 2012 cameras can only deal with three groups in the time between shutter press and shutter release. I believe this is why pre 2012 bodies have ETTL and HSS, Group mode, and sync limitations, they were not designed for more than three group functionality.
Of course if Canon wanted to they could probably firmware upgrade the pre 2012 bodies to work with full five RT groups, but I suspect even the post 2012 bodies would suffer shutter lag and very possibly sync limitations with an eight group ETTL firing sequence.