That means I could use illusion on a legendary and then bring it into battle.
That also means that ANY Pokemon can be made as an illusion, not just mine.
This doesn't fall in line with the original game mechanics of Pokemon and could probably be exploited.
Abilities are usually not used outside of battle. Moves are.
Model Rater, Pixelmon Tester, Official Forum Supporter, Gym Development Member!
Not the Pokemon we need, but the one we deserve.
Personally I don't think such a purely cosmetic feature (that hardly anyone would ever use) is worth the amount of work it would take to implement.