A point I forgot to include in my last post is this. I think companions are a good addition to the game. They are fairly well balanced. They offer combat support to players who need it and an additional layer of depth to the game as a whole. On the topic of this thread, I doubt they are making anyone a better player combat wise.
For me, Bastian has not improved my playing ability, to be honest he's a bit of a crutch.
However, he's useful in tougher fights (e.g. when I occasionally foray into group dungeons). He dies rarely, and occasionally distracts my enemy(ies) - only occasionally, though. To get him to this stage, I had to fully level him up, and equip him with epic-quality equipment and weapon.
As a small bonus, he occasionally comes up with memorable combat phrases e.g. "Mess with the swordsman, you get the point!".
PC(Steam) / EU / play from Melbourne, Australia / avg ping 390