Using the Sinestro Corps in Green Lantern Corps also opens the door to adapt Parallax, the fear entity, correctly this time around.
Mark Strong’s Sinestro never had the opportunity to fully cross over to the dark side, so Green Lantern Corps shouldn’t wait on making their Sinestro a villain.
While most of the Corps are filled with despicable creatures, Sinestro sees his Corps as the only way to instill order throughout the universe, something he believes the Green Lantern Corps failed with.
So in his eyes, he sees his Corps as a force for the greater good, making them the perfect foil for the next Green Lantern movie.