All the photogs I know who have the 600EX-RTs have been waiting for f***king ages for Canon to release a 430ex-RT. I would much rather have a Canon Speedlight than a 3rd-party option, but since they seem not to have any interest in releasing a mid-range RT-flash they left the door wide open for 3rd-parties...
I already have three 600's and I'm not willing to fork out another $1,300 for two more, that don't need to be that powerful.
I have three 600s as well (and an ST-E3-RT), and I find the extra power over the 430s I previously had useful with modifiers. It's also nice to have the same power level on all the slave units.
How often do you use all your speedlights at max power at the same time? I can see the need to do that sometimes but for me not that often. I can't speak to others needs but I can get by with having 3x 600s and two less powerful flashes. I can always use an assistant with a reflector to bounce light back into the scene, but that's just me. I'd rather see my money go towrads getting a couple of monolights instead of two more 600s.
Some of us can't afford assistants!!!
Those tend to run more than flashes (purchase vs salary).