Qui-Gon Jinn: Obi-Wan, promise... Promise me you will train the boy.
Obi-Wan: Yes, master.
Qui-Gon Jinn: He is the chosen one. He will bring balance. Train him.
Plus, wasn't Palpatine busy being melted by Windu when Anakin chopped off his arm (I'm fully prepared for the "actually Palpatine was in complete control" response)? It seems to me they have a better chance of beating Palpatine if he doesn't have a chosen one on hand to kill loads of Jedi. This is my whole problem with the "prophecy" stuff in the prequels, it reduces the whole thing to a tableau and fait accompli - Vader doesn't kill the emperor out of reawakened parental love but to fulfil a prophecy. Similarly, Luke doesn't throw his lightsaber down and refuse to fight out of compassion, it's because otherwise the prophecy wouldn't be fulfilled. Agency is important in a film series about emotion, which is why people hate the prequels so much aside from the dialogue and Jar Jar.
McGregor plays him well, but what is there to the character that wasn't there with Guinness? He barely changes from the time he's an apprentice. Would've been way more interesting to have him be headstrong and play a bigger part in Vader's downfall, as he implied in RotJ.