I do hate to be a nitpicker, but I don't think Omega was a summoner. Auron (or Wakka) stated when you first walk into the Omega Ruins that, the traitor Omega, was a MONK who betrayed the teachings of Yevon (not Yu Yevon, the religion Yevon) and was exciled for it.
I'd have said a Summoner is a monk or nun.
And the reason (I think) he didn't keep his original form is because he knew what Yevon did. If Yevon did it, he could do it too. So he did and became Omega the beast, who was just considerably weaker than Sin.