The handling charactoristics of a trainer have little to do with flat bottom airfoils or semisymetric airfoils. Dihedral, wing loading, CG location, tail moment, thrust angles, etc. is what makes a trainer a trainer. I'm pretty sure you could build a trainer with a perfectly symetrical wing. I think most trainers have flat bottom airfoils because there used to be a time when there was no such thing as an almost ready to fly or ready to fly model airplane. If you wanted a model airplane, you used to have to buy a kit and build it yourself and flat bottom wings are simple to build.