I was tempted to reply that if you don't need f2.8 for either low light, subject isolation, or AF with TC's, why on earth other than ego, would you carry it around - assuming there was a viable f4 alternative of similar IQ?
But, then I remembered: You get a noticeably brighter viewfinder from f2.8 than from f4. That is a real practical, as well as a comfort, advantage to a lot of us. Once you get used to that, it's probably hard to give up. Also, all other tings being equal, AF should be faster with more light from the f2.8 lens. I don't know that for certain, but it's consistent with what I do know about how AF diminishes with poorer light.
I know that when I put away my fast f1.8 primes after basketball season, and break out my other lenses for the summer shoots, I miss the brighter viewfinder.