Indeed.
I actually want an f4 to complement mine, just for the weight savings. As a matter of fact, after beginning to use my 6D FF 70% of the time, I've a good mind to just sell the 2.8 and buy an f4 altogether, but I'm not quite there yet.
I think it's really all about whether you really *need* the 2.8. If you shoot indoor sports often, then the F4 is not a true substitute.
If you are shooting portraits and want the narrow DOF.. some will tell you that you need the 2.8, but I've seen professional portrait photographers stick with the F4, or the F4 and a couple primes. The slight difference in DOF when shooting telephoto isn't necessarily worth the extra bulk of the 2.8.
I'm shooting Nikon.. For the same price, I could have gotten the 70-200/4 from Nikon, or a 3rd party 70-200/2.8 (the Tamron is supposedly pretty good). But I chose the F4 for the weight difference. I can't realistically see myself carrying the 2.8 often.