For the most part, I keep 1-2 of every pokemon I catch and replace them if I find a stronger version of said pokemon. I do hold on to copies of some pokemon to evolve for EXP, like Pidgey. Pidgeys only cost 12 candies to evolve, and they pop up even faster than McDonalds locations in the USA. Hold on to 20+ Pidgeys, get enough candies to evolve them all, then put on a lucky egg and enjoy your 20k+ exp.
I have many pidgeys and a couple lucky eggs. Thanks for the advice. I should be able to get to lvl 13 with this. Nice.