A significant savings for block mode occurs if the terminal is connected to its host via a network. With block mode, a large number of characters are sent in a single packet. While block mode is more efficient, it is nearly extinct, and for good reason. Faster and cheaper computers made the higher efficiency less important. Another point is that the efficiency is not of much significance where the user doesn't type in very much.
Editors are a primary example of where the user types in a lot.
But if you use block mode for editing, you must then use the crude editor built into terminal. Modern editors like vim and emacs are much better but can't use block mode. The terminfo data base doesn't seem to include it and this would complicate writing software for it. The RS-232 Solution, 2nd ed., Sybex, 1982. RS-232 Simplified, Prentice Hall, 1987.