Call Wyvern was originally DRG's SP ability and that's the primary reason it's not subjob accessible. A half level (okay, now slightly more than half level) wyvern isn't going to be anything to write home about) anyway. It's also in line with no /PUP puppet and no jug pets and useless charm on /BST. /SMN is slightly less unusable but the summoning magic is basically the only thing you get from the subjob in the first place aside from auto refresh (and odin/alexander are unusable)