I go back and forth when it comes to Rafinha. I want to like him, but it's hard to do so when his profile as a player is so visibly defined by his limitations. He's an incredibly one-sided player. He only balances himself so that he can have the ball on his left foot to carry inside. It's always those identical tiptoe dribbles cutting in as well that can easily be shackled under the right pressure. Add to that his fairly obvious lack of vision (generally avoids passes that cut through markers, short or long) and it makes his playing style predictable and only good for certain scenarios. It's like he has one very small and specific set of actions that he does in a match, outside of which he makes little to no contribution.
That lack of options would be alright if he was a right winger (he'd have to be better at his few tricks tho) but in midfield a player is responsible for a field of vision that extends in all directions. Rakitic may not be a technical wizard that excels at all aspects of midfield play but at least he's not a one-trick pony midfielder, same goes for Roberto. They are support outlets in a system and do the many duties required of that role on and off the ball.
Rafa is still growing as a player and he obviously deserves a chance to show his abilities in a larger sample set than one season but I've personally kind of gotten accommodated to the idea of having Samper groomed as a central midfielder while Roberto is groomed for the utility/rotation role. They both have roles in the squad that they can fulfill and grow into, whereas Rafinha really doesn't have one. He doesn't have a versatile enough skillset to become a reliable utility squad player in midfield or the special 'talent' to grow into a role where the team would be built around him. That's why I want as many of the rotation minutes as possible to go to Samper and Roberto, because in the 2nd half of the season there'll be less rotation and possibly an extra midfielder (transfer) to compete with.