I guess the appeal is in seeing how heroes would behave if they were unburdened by their moral code. Actors often say that villains are more fun to play because they don't have to follow the rules like heroes do. Having a villain become good is like seeing that character becoming shackled by a moral code which would take the 'fun' out of that character. In G1 we had the episode "Attack of the Autobots" which showed the Autobots becoming evil. "Megatron's Master Plan" showed the Decepticons being good, but it was all a ruse. BM Jetstorm was an evil version of Silverbolt.

We see it other franchises too, e.g.
+ Red Kryptonite can remove Kal El's inhibitions causing him to become incredibly selfish and even megalomaniacal.
+ Toy Story = seeing Emperor Zurg turn good and become Buzz 2's father in TS2 was cute and funny, but not as exciting as seeing Buzz become Lotso's minion in TS3 (then becoming Spanish Mode Buzz was hilarious )
etc.