The point of not evolving and saving for later is that after level 15 or so xp starts to step up quite a lot. 39 to 40 is 5million exp.
Anyway, imho it makes mid levels to go fastr, but actually it doesn't change much when you use them. The point is to use a lucky egg and evolve all the pokemon you can. I missed the train already, so i'm just 17, but i feel it's not much an issue.
Level counts only for how strong you find pokemons. Gyms are a joke as attackers simply because you can spam it so even the strongest pokemon is not a guarantee to keep it for a long time.