GL decides canon with popular sentiment, it completely changes to suit the next target market.
Originally when ESB was made GL said about the Emperor's appearance that he was once human but had been twisted and mutated by his use of the Dark Side of the Force. This was carried through with even more emphasis in RotJ and was originally intended to mean by exploring the Dark Side over an extended period of time had turned Palpatine into a mutated near-human visually representative of daemonic possession in superstition.
During the making of the prequels it is a younger, newer target audience and classical ideas about religion are not as popular today as they were then, the Exorcist and Amityville Horror are pretty lame by today's standards, no matter how terrifying they were then. People like to see mechanical systems at work now. If somebody has superpowers they want to see the biochemical reason for it. If they look possessed they want to see the medical event which caused it. You just won't get any suspension of disbelief otherwise, where most of my contemporaries still claim they see ghosts and genuinely believe in the supernatural.
So canon for the dark side transition changed since the Star Wars saga began.
For the OP I like the thinking of having reasons for dark side transitions. I do rather like the original ESB era thinking of a daemonically possessed human being simulated by falling to the dark side of the Force, kinda suggesting that's where less advanced civilisations come up with that superstition without understanding the Force.
Some hint of why the Dark Side works that way is still canonical. The Jedi Knights do not let themselves be controlled by the Force as a power per se, as this would be to let their base desires have free reign. They use the Force only for knowledge and defence, as a tool (excellent example is the lightsabre, a proxy for the Force rather than say using it to shoot lightning from their fingertips). Dark Siders attempt gather as much of the Force as possible into themselves, regarding it as a means to personal power.
I think this kind of use of the Force is why dark siders are visibly transformed. It is the maxim that absolute power corrupts absolutely. The Jedi purposely keep a distance, using the Jedi Code and the Lightsabre.
It was a mistake for GL to explain Palpy's appearance the way he chose to in Ep3. But then there's a number of things about the ending of that movie which scream comical pase and almost manage to undo a lot of previously intriguing work. I think he listened too much to the marketing analysts who told him today's cinema going youth prefer to see karma than daemonic possession, and he altered the theme of the Dark Side accordingly.