It was added as something extra to encourage more moving around. Capturing more of the same breed is still the best way to build candy. Magikarp are everywhere, you'll get enough eventually.

Playing Pokemon Go made me crave the real game again so now I'm busy breeding up perfect Greninjas to be as interested in this. I did evolve a Blastoise the other day and now I'm up to level 23 with 123 pokemon captured.