We really just don't want to bloat the summon monster tables with too many choices, honestly. It's already kinda hard to choose a monster; making there be TOO many choices spread across TOO many books just leads to option paralysis in game and that's not good.
Adding new monsters to the summon monster lists is a great house rule. And we add a few here and there in the deity articles we print every four volumes in Pathifnder. But beyond that... we are unlikely to officially expand the summon spells with new Bestairies. Just not a direction we're interested in going.