Nobody has ever cared about it except for presumably a developer or two who thought making writing on the walls with them would be entertaining.
Useless in battle.
Most of them don't even really look like the letter they're supposed to be.
The developers have to make 28 sprites/models, animations, rigs and textures for one Pokemon nobody cares about. Sure they wouldn't be hugely difficult to do but for a Pokemon nobody likes its a lot of work.
Can't even make a team of them spelling out a funny six letter word because you can't use more then one of the same Pokemon in any serious battle.