I've never used such screens, but do appreciate they could be very useful at times - rare times for me.
I see two potential problems with implementing them on higher grade equipment:
a. I can't see anybody with real photographic experience, working off-tipod, who'd welcome using a rear screen to frame and/or focus; reducing a camera to the norms of a cellphone seems absurd, not a particularly helpful "advance";
b. as somebody mentioned, all that articulation can not avoid putting wear and flexing onto components. That's never a good idea. And as pro cameras are supposed to be there to get a lot of heavy use, and to be dependable, any structural weakness won't market well.
As ever, there are bound to be exceptions. But again, for me, no thanks; the few missed shots don't outweight the greater dependabilty question. I mean, Elliott still got his pooch shots, didn't he?
Rob