That's fine as a rule, but i also think that difficulties should always be named in a measurable way, or something that has a start and an end (like the Seed > Sappling > Tree that someone mentioned) as long as the player doesn't need prior knowledge of the source to know which diff is which.
The reason difficulty names can be customized is for Guest mappers' names and for songs that come from other sources with a different difficulty naming (like DDR's Light, Standard, Heavy, Challenge) to allow the mapper if they so want to use the difficulty naming of the game (difficulty naming not character naming, nor spell naming, nor level naming, etc).
The reason difficulty names can be customized is for Guest mappers' names and for songs that come from other sources with a different difficulty naming (like DDR's Light, Standard, Heavy, Challenge) to allow the mapper if they so want to use the difficulty naming of the game (difficulty naming not character naming, nor spell naming, nor level naming, etc).