Well Rubio isn't a shooter or a scorer and yes he's better than their "PG" last year, but that's not a hard thing to achieve when you realize they didn't have one. The problem is Rubio is a passer and a decent, at best and on a good night, scorer. On offense, he needs the ball to be effective because he certainly isn't a spot up shooter.
The issue lies with his fit with Booker, whose best skill is 1 on 1 type of offense and creating relatively well for others (though he isn't the most willing passer). What do the Suns do? Give the ball to Rubio and relegate Booker to a spot up shooter/cutter? I wouldn't. But what do you then with Rubio? Zone him out of plays and relegate him to spot up shooter status? With his shooting percentages? The answer is **** no. They don't pair well with one another at all. Ideally, you would want a decent playmaking PG with a history of playing good defense while also being a good shooter to pair with Booker. Rubio.... isn't that. He will help on defense and moving the ball, but he's got the same issues as Rondo; he can't shoot (.322% from 3 for his career, even lower than that last year on nearly 4 attempts per game).