I've yet to see anyone explain why 24-bit colour is an improvement
Because it will support a greater range of colours, simultaneously and independent of a users custom colour settings and any 'standards' that are associated with the current range. Tah-dah! Perhaps it isn't an improvement *to you*, but the fact you find yourself making the same points is probably a good indication it is to others.
/me gets a profound sense of deja vu