Exactly why I went for the 600 f4TC. I was shooting at 600mm+ all the time, my most used lens was the 800PF so, when I was deliberating on whether to get the 400 f2.8TC or the 600 f4TC, the longer lens was it.My go to long lens for low light is a 600mm f/4.
Many prefer the 400mm f/2.8 but when I owned one I always had a TC mounted as 400mm is a bit short for my typical subjects.