It's just that at some point you run out of letters, having some 35 glyphs more just comes in handy. Very much so if you have indices for different spaces. Not

It's just that at some point you run out of letters, having some 35 glyphs more just comes in handy. Very much so if you have indices for different spaces. Notation shouldn't be a problem if it's compact enough. Then the bra-ket one is foolproof and not too many physicists study functional analysis proper, so it makes sense to simply follow Dirac there.