Well, the only thing that you will really gain by not letting your Treecko evolve is the ability to learn Mega Drain and eventually Giga Drain. And of course, if you don't let your Pokémon evolve, they will learn moves at earlier levels than their evolved forms. However, Leaf Blade is a better move than Giga Drain IMO, and if you wait too long to evolve Treecko, you will have to use the Move Tutor to get Leaf Blade. If you delay a Pokémon's evolution, you aren't going to make it any weaker, it will still be getting its EVs. IMO, it would be best to just let your Treecko evolve naturally, (at Level 16 into Grovyle, and at Level 36 into Sceptile) but you aren't going to hurt anything if you decide not to.