Anakin still did a pretty good job of wiping out lots of Jedi. And he kicked Tyranus butt with barely any assistance from Obi-Wan. So pre-injury, he was a pretty solid sidekick, well on his way to becoming a Sith Lord Extraordinaire.
After his injury, Sidious knew that Vader couldn’t do all those fun tricks like microwaving popcorn with his fingers, but he was still a pretty tough guy. And with no other Jedi floating around, Vader could still be very effective. All the guy had to do was use the force to choke the crap out of people and deflect a laser blast with his light saber to convice people that he was not a man to be trifled with. So why not keep the guy around to scare the crap out of people.
Beats going out and training a new apprentice. It took ten years from the time he first identified the kid to bring Anakin around.
I think if they had realized that a lot of the Jedi Knights had been killed by Stormtroopers, they would have never taken his shit and sent a bunch of Stormtroopers to kill him in the bathroom. And if they found out that he was really just a whiny little bitch with a weakness for Natalie Portman, Vader would have been a joke. The guys at the Death Star officers club would have been sending him blow up Amidala dolls for his birthday.