I went in the one by the baboons once, to check out the baboon viewing and the cafe inside. It was absolutely heaving, and on a not-especially busy day - I never even made it near the window. To be honest, and much as I hate to admit it, they quite possibly do need more under-cover space. Just console yourself that they're building for Giant Otters at the same time.
They don't "need" one,but they get lots of yearly memberships from young families who come just to use the play areas rather than pay £4 or £5 each time to use a soft play facility locally. It also brings in lots of revenue in the quieter months especially in the cafes.
I'm not a fan as I go to wildlife parks/zoo's to see animals. But if it keeps the kids happy then the parents are going to be happy and come again. So everyone's a winner.