I have planes with and without a airfoiled tail

With: Sig King Kobra, Goldberg Falcon 56

Without: Sig Kougar, and many others.

The one thing that I have noticed is at slower speed the planes with the airfoiled surfaces they have less of a tendendency to "Snap out."

To the original question. IMO the reason trainers don't have the airfoiled tail surfaces is cost. Especially in the "old days" Before the availability of "Arfs"