Roberto was much better than him as rb.
It was easy to predict.
In Barca, fullback probably attacks and links up with teammates for 70% of time (while we have the ball) and defends only 30% of time.
This is why guys like Dani Alves play awesome, because they are better midfielders/attackers than defenders.
The same answer is why Busi/Roberto/Samper are better defensive midfielders than Masch, even though Masch is a better defensive player than any of them.
But if our defensive midfielder makes 20 passes/links up in 45 minutes, and needs only 3-4 defensive actions against average La liga team, then it is clear why we suffer with a defensive midfielder like Masch, or with a full back like Bartra.
This is one more example why we can't play with one-dimensional players (except on central defender's position).
All of our fullbacks, defensive midfielders, midfielders, wingers and attackers need to be both technically, tactically good and awesome in passing, vision, creation, linking up, off the ball movement etc.
This is why we will always slightly suffer and play with a 10 men, when:
1. Bartra/Montoya play as a Rb
2. Mascherano plays as a defensive midfielder
3. Mathieu plays as a Lb
4. Gumbau is on the field
5. when Adama is a winger etc
But at least a part of our fans will stop asking for Bartra's chances at Rb.
He is not good enough in technical, attacking, linking up part.
He isn't any attacking threat, and that is a problem since we always have a possession 60-70%.
In those moments, the opponent's aren't marking Bartra because he is not dangerous at all, and that also makes a job harder for Messi and Rakitic on that side.
Because when Alves (or even Roberto) play, the opponents need to mark 3 of our players on that side who are trying to create a mess on the right side.
But when Bartra/Montoya/Douglas are there, defenders are marking these guys much less, and the opponents have an extra man to mark Messi.
So, more or less, a player like Bartra on a RB means an extra defender on Messi, and our whole team is suddenly crippled.
So, for Barca, it is much more important to have a dangerous player at a RB, like Alves, who will be marked by at least one defender.
That leaves more space for Messi. And when Messi has more space, that usually means more goals by Messi, more deadly passes to Neymar and Suarez etc.
So, sorry Bartra, but you will be our 5th defender and that's it.
The balance of the whole team is much more important.