^Well, the TAS producers wanted elements that were visually striking and impossible to achieve in live action. If you've seen Bantam's Star Trek: The New Voyages 2 anthology, it contains a rejected Russell Bates script, "The Patient Parasites," which was turned down because it didn't have anything in it that couldn't have been done in live action. Bates went on to team up with animation writer David Wise and write "How Sharper Than a Serpent's Tooth," an episode with a winged-snake alien "god," a vast holographic city, and a menagerie full of alien beasts, so he definitely learned from the rejection.
Indeed, as I suggested before, that makes it strange that "Albatross" sold at all, given that it has nothing (aside from the design of the aliens) that couldn't have been done in live action. Maybe it was the strength of the story that sold it despite that.
To get an idea of what The Patient Parasites might have looked like, check out the animated series comic I produced with Mr. Bates full co-operation.