C scores are “useful” in that sense, if you never bothered to learn how to read anything else. (This is one of the drawbacks of easy access to computer playback, and “composers” who have never interacted with live musicians, IMO!)
In the baroque era, keyboard parts were often written using C clefs (on any of the 5 lines of the both staves, to minimize the number of leger lines), or even in open score for contrapuntal music - and nobody at the time thought that was too hard to read. (And since most people manage to learn the basics of their native spoken language at a very young age with no formal instruction at all, it’s hard to argue that learning music notation is objectively “hard” in comparison!)