Rapptor - right decalage has nothing to do with CG - BUT - CG determines the airplane pitch stability because of its relation to the Neutral Point. Decalage determines the angle of attack stability of the stable airplane. You want angle of attack stability in a trainer and a strong pitch stability due to CG location.

Using trim or some other trick to get good flight isn't a sign of a poor design - its a sign of an airplane designed for a particular task. If the task is lowest trim drag at the most efficient crusie speed of the engine at a particular altitude then any trick or trim device you can drag up is fair game and will make it a good design.

When the altitude changes - then the criteria has changed and of course things need to be rebalanced aerodynamically.

For a trainer a symmetrical setup isn't too good since the angle of attack stability will help keep the airplane level.

That leaves up with the symmetrical setup is good just for great pattern flying airplanes - or anything that is going to spend as much time upright as inverted as flying sidewise and in snaps. Any of our hovering firebreathing monsters will work better with symmetrical as will the 7 ounce indoor mini firebreathers.