If you can manipulate your companions into becoming different personalities taking significantly different paths, they are weak characters.
I for one don't want to feel like a puppet master, deciding whether Gale and others will be taking a dark or a redemption path this time around. Influence them within reason - yes, puppet master - no.
There is such as thing as giving the player too much control over the story and characters. The characters need to be well motivated, well written and believable more than puppets for the player to play with.
Just so you know, way back as far as BG2 you could change both Viconia and Sarevok's alignment from evil (neutral evil I think for both) to chaotic neutral. Whether you like it or not it is a long established feature of character rpgs which are built upon reactivity
