An efficient light weight lighting kit is more valuable than an expensive heavy lens that you will rarely use.
I guess I'm not sure how two or three 600EX-RT flash units along with possible stands, light modifiers and batteries can be considered "light weight" compared with a lens but you're right if compared to a bunch of studio strobes. As for subjective value or assumed rarity of use compared with a lens, my flashes come out a lot less than my lenses in the types of walk around journalistic shooting I do. And these large flashes are actually larger than most lenses, the 70-200/2.8 being one exception. (I use a small RD2000 flash most of the time but I would love to have something similar in size from Canon that was controlled ETTL wirelessly directly out of the camera.) YMMV. My flashes would be used a LOT MORE if I could control one off-camera flash straight from the camera and not be forced to have a big ass flash attached to my hotshoe for the sole purpose of controlling another big ass remote flash that was the only one I wanted to use in the first place. (Carry two large flashes just to use/control the single active large off-camera flash = stupid PITA to me.) The ST-E3-RT controller is about the size of the RD2000 flash I use now but still requires me to hand carry the 600 flash because I can't mount the 600 anywhere. That's dumb. At least put a hotshoe on the controller. But no! Find a pocket, backpack or between your knees for that flash when you can't hold it all the time in your other hand when focusing, etc.
Hence the natural thought progression of ... lose the ST-E3-RT controller all together and put that functionality into the camera itself. Now just carry one flash and let it live on the camera when not used off camera. One flash, multiple uses. Simple but for some reason not within Canon's grasp to offer.