Imagine giving Myriad to an Avenger of Zendikar
That's just scratching the surface, and it's worth mentioning that, even if it turns into another creature for the turn, Scion still doles out commander damage
Knowing that Pidgeot ex's Quick Search ability can only be used once each turn, even if you have multiple copies in play, will help you plan out your turns more efficiently so you're not accidentally breaking any rules or cutting yourself off from different lines of play
Cresselia ex has a decent attack in Psychic Flash, which deals 80 damage for three Energy, but the Lunar Plumage ability allows it to heal 20 HP each time you attach a Psychic Energy to it
Gods do not show up in every plane and set, but when they are featured they are integral to a sets mechanics and story
That said the card would not see all that much more play if were a guaranteed 5 damage every time (counter burn being the only deck that this would be really significant in)