Originally Posted by Giratina
X needs to be a human because being a little blob with an eye isn't quite as effective as having the ability to punch and kick and not try to be captured by every unknowing Unown/Trainer they met as well as some of your powers (I personally thought that X and Julia would keep some of the powers they had as Unown, hence making them the superiors - matriarch and patriarch, if you will. Only those two, though; Rodney can communicate with Tristan through Psychic Pokémon connection, without speaking.)
Still, if all you know is being an Unknown and have mastered your powers in that form why would any unknown want to change their body into something totally alien to them and have to learn things all over again (or in a worst case, lose most of your powers)? Why not just have the battle in you own forms in your own world and watch all the humans look on helplessly as they have no clue what is going on ;)? If they can punch and kick or have pokeballs makes no difference if they can never find you to attack you after all..To say nothing about how effective unseen Unowns' powers have been shown to be against humans to start with...