Let's be honest. Look how Ash do as a trainer:
He has travelled through 4 regions (Kanto, Orange, Johto, Hoenn) and is currently travelling through Orre, his fifth region... And how many Pokémon have he caught? Well, he got 51 Pokémons, but 30 of them are Tauroses...!
And he gets rid of his powerful Pokémons all the time! Charizard, Squirtle, Beedrill, Pidgeot, Primeape, Haunter, Butterfree, Lapras... Of course, he had different reasons to do so, but he would have been twice the trainer he is now, if...
No, he wouldn't. Cause he would just leave them at Oak's lab and catch a few new weaklings to train! Sceptile, Donphan, Swellow, Glalie, Bayleef, Bulbasaur... Why? Why do he insist on leaving all those powerhouses at Oak's lab, and not use them to be succesfull as a trainer?
Cause he sucks. Ash can't train Pokémon. Ash can't battle. Ash sucks.