Aesthetics argues against that, as well as tradition. (and you failed to note the use of primes and double primes)
It's also a meaningful differentiation to be able to communicate a sentence such as:
“’struth,” he said, “she’s 5′2″ with eyes of blue.” or, “Usually, 35–40 m.p.h. is a workable and safe speed for travel—unless pedestrians are involved, in which case adjust by −20 m.p.h.”
which are far less intelligible when set with unidirectional/same width characters.
It's also useful to have the uni-directional versions ' and " for special usages in computer science contexts, while the curly forms can be used within say a .csv without the need to be escaped.
It's also a meaningful differentiation to be able to communicate a sentence such as:
“’struth,” he said, “she’s 5′2″ with eyes of blue.” or, “Usually, 35–40 m.p.h. is a workable and safe speed for travel—unless pedestrians are involved, in which case adjust by −20 m.p.h.”
which are far less intelligible when set with unidirectional/same width characters.
It's also useful to have the uni-directional versions ' and " for special usages in computer science contexts, while the curly forms can be used within say a .csv without the need to be escaped.