A Snorlax with 0% IVs and the worst defending moveset (Lick + Earthquake) is still going to do a decent job of defending just because of it's massive HP.
You could argue that no pokemon is worth powering up because gyms are so easy to take down regardless of what pokemon you put in even with their best movesets, and because gyms are so easy to take down, you don't need fully powered pokemon to do so.