Barca 70% possesion -> Alves 70% offensive, link-up duties.
Opponent 30% possesion -> Alves 30% defensive duties.
What you are saying is that you want Alves to be defending when we have the ball and that's so uncharacteristic for Barcelona.
It's inevitable that Barcelona being one of the better teams and with a possession based philosophy are going to spend more time attacking than defending.
However, if a crap team spends most of it's time without the ball and defending it's goal, does that also mean that their Strikers primary role is as defender? NO, it doesn't. He is expected to score goals and attack the goal. Same goes for a Full-Back, who falls under the Category of Defender btw.
A Full-Backs role has changed significantly over the past 30 years and I agree that they are a massive part of a teams attacking arsenal nowadays. But his First Objective is to Protect His Goal. This should rank above everything else.