F/2.8 zooms are vital. They're 'do everything' lenses that do a decent job of almost anything, without requiring lens changes - a photojournalist's or event photographer's ideal tool. To that end, they are optimised for versatility over other considerations - they can isolate subjects, but not like a prime, and they are fairly sharp, but not like a prime, and not like an f/4 zoom could be. They are ideal for a role that requires this versatility in a single lens but does not require ultimate sharpness and can manage with big, heavy lenses.
Canon are on to a good thing with their f/2 zooms. Huge and heavy, they take what an f/2.8 zoom can do and do it even better, taking subject isolation to prime-like levels (if not equal to the widest primes) at the cost of weight. For certain wedding photographers, this may be acceptable.
High-end (as opposed to budget-oriented consumer-end) f/4 zooms are different, despite sharing similar focal lengths. They are likely to be used less by the always-ready, shoot-from-the-hip event photographer's or photojournalist and more by those more deliberate, slower-paced photographers who would like a prime's sharpness, but don't need the wide aperture (at least at most focal lengths) and could use a zoom to be able to shoot at a range of focal lengths instead of carrying a bag of primes, since they're rarely using the prime's aperture and subject isolation ability anyway. Unlike consumer zooms, which have a strict price constraint and need to be a one-lens-does-most solution for those who are only ever to bring one or two lenses, these zooms would be designed with much looser price constraints and an emphasis on image quality rather than large zoom range - users of these lenses are likely to be willing to buy and carry several lenses, unless going ultra-light for a specific outing. Like the f/2.8 zooms, they would also need good dust and weather sealing, since they'd commonly be used in adverse conditions.
Beyond the obvious aperture difference, the key difference between a f/2.8 zoom and a premium f/4 zoom is the difference in design focus. F/2.8 zooms are meant to shoot everything and anything, with minimal lens changing, but without necessarily being the best at anything. F/4 lenses would be much more focused - designed to excel image quality wise, with the ability to replace primes within the limits of their aperture range (or at f/5.6 and below, at least), without any pretence of also being ideal for subject isolation. They would likely supplant the slower primes (say, the Zeiss Batis line) while complementing other, faster primes, allowing a user to have prime-level image quality throughout the zoom range, while adding one or two primes for subject isolation or low-light capability at the focal lengths where they are really required.
