Similarly, Gardevoir, who in my opinion outclasses Mr. Mime, is already available unlike Mr. Mime. I think Caz has his reasons, though.
On the other hand, I'd say that competitive viability is not the only factor taken into account when decided which Pokemon are available (and when they are available). Galvantula has Sticky Web going for it (and should you have Choice Scarf or just EV train it's Speed) you can pretty much always set Sticky Web in Gym Leader battles. However, I recall Caz saying Double Battles are becoming more frequent from now on.