It would be cool if, for example, placeholder %player_name% was replaced by the player's nickname.
It would be useful when using the In-game Account Switcher mod:
/gamemode creative %player_name%
/gamemode spectator %player_name%

You can generally develop this whole topic and add many different placeholders that would be useful for entering commands.

The example you give is already in vanilla as @s.

This does not work on servers...

The example you give is already in vanilla as @s.