It was useful to me when I made Diamondus V (the vector style edit of diamondus) for one of my SP episodes.

JJ2 has a funny way of doing palletes - if a colour does not appear in the tileset itself, the game will assign it to its default value. Thus, if you want to make a weird pallete edit, you need to make sure every colour appears in the tileset. Diamondus V is weird in that the majority of the pallete is the same colour (black), and the rest of it is taken by 7 colours (the tileset only has 8 colours in total, including the pallete. 9 if you count transparency). Long story short, it has A LOT of duplicates. In order to get the pallete to work, I needed to make sure that every single one of those duplicates was in the tileset itself.

This is literally the only case I can think of where there was a reason to do this, though. Normally it's not worth it.