Not all Pokémon can understand us, and not all trainers give vocal commands to begin with. There are other ways to work strategies out with Pokémon that don't involve giving things away by shouting out attack names constantly. Besides, you'd be surprised how well they can battle by thinking on the fly.

[But this is less about strategy and more about understanding... sigh.]

Besides, I know for a fact that my Aerodactyl can't understand me at all. The others can, and it's not like I spout insults at her, but she's never understood words. Ever.