Blake is not a better one on one player than Jack; Blake has no skill demonstrably better than Jack. Career 3-point averages are meaningless - recent performance and trends are far better indicators of immediate future performance and trends. Even if Blake gets better at some skills, with his one potentially valuable skill being 3-point shooting, Jack is likely to improve faster and be better.
So why Blake again?
Because the Blazers do not trust Jack to run the team like it should be run - which would threaten the development of the rest of the youngsters. If Jack doesn't start, it won't be because he isn't the better basketball player, it will be because Blake will be a more stable, error-free guard. Blake will play the game McMillan wants him to play, while Jack - in my opinion - is more prone to play for the moment.
From a pure performance perspective - Blake is a backup point guard, but he doesn't think he is a backup. Many fans don't believe he is a backup. But lets face it, Steve Blake is a solid backup. What is being over looked is Sergio Rodriguez will be the point guard of the future, and Blake will help Sergio hone the skills necessary to run the team.