Trainers need to be good handling planes to keep the student's workload down as much as possible. Training aircraft which have a lot of adverse yaw, roll couple, and pitch couple are difficult for anyone to deal with let alone a student. The student cannot tell whether the plane's bad behavior is his fault or the planes fault. In other words, if a plane handles like a well tuned aerobatic type, with very little cross coupling in the control systems, it is easier for the student to figure what he/she is doing right or wrong.
That being said let me say that the single worst handling trainer I have ever seen is the Perfect Trainer by Great Planes. It has so many ground and aerial issues I do not care to list them here. The next worst handling plane I have seen which is sold as a trainer is the Nextar. All the add -on's to solve problems do not help.
In both cases it looks like the designers patched a bad handling plane every way they could without solving the basic problem of poor initial design.
The good part of the experience is that it has inspired me to think outside the box in designing my own trainers to get a far more positive result. How about a plane that uses a rectangular wing with fully symmetrical airfoil and no dihedral? My view of what constitutes a good trainer design is far from the conventional way of doing things.
But then its was the really poor flight characteristic of the many trainers I have flown that has encouraged me to look for something better. Art Cloutier