It's nothing special. My Mum joined when it first started, and although we put down that we had 2 rabbits, 2 cats and 1 goldfish (coz thats what we had at the time) They still only ever send us the same vouchers as everyone else.
It's good if you have dogs & cats and if you spent LOADS of money there (I'm speaking £50+ in one go) as thats the only vouchers we got, and My Grandma would get the exact same ones too, and she only has 2 cats