Aerodynamist friend once explained that in full scale sometimes thick trailing edges are used to get more effect out of control surface without increasing control force as much. Aerodynamically control surface area seems almost as large as it would if it continues to sharp point. And the effect is stronger in slower speed where more control is usually needed like landing and T/O.

If you need more rudder and your model is already 2M you can install triangle spar to the trailing edge. I once had as wide as 12mm almost 1/2 inch and didnt notice anything bad behavior. Yrt knive edge and stall turns got better.

Structure stiffness versus weight is better when thickness ingreases.