I like to imagine that Zira doesn't totally see Nuka as useless to her pride. Being the only adult male in their pride, he has other things he can offer, it's just a matter of getting him interested.

But because she's always brutally mean to him, he doesn't trust her sudden, gentle side when in heat. At first Nuka's reluctance to breed with his mother and sisters is an annoyance, but over time he comes to see it as a way to get "love and attention" from his mother. It is the only time she lets him be affectionate, so he takes what he can get.