Well, if the bench never plays, they get too rusty. Also, Valverde needs to see them in action, so he can have an accurate, current understanding of how they play.
We continue to see that the two most important players, by far, are Messi and Suarez. Without them, Barca might be mid-table in La Liga, definitely not in the Champions League.
Rakitic and Busquets are the best Barca midfielders. There are good things about Arthur, Vidal, and Alena, but their quality is currently significantly lower than Rakitic and Busquets.
Barca has one player who can play LB. Semedo put in a big effort yesterday, but the Sevilla wing players dominated that side of the field.
Coutinho is in a bit of a funk currently, but this team has no choice but to continue to play him whenever all three of Messi, Suarez and Dembele are not available. The talent level drop from Coutinho to Malcom and Boatang is so huge, Coutinho just has to play his way into better form.
The rotation will be tightening up now, or very soon. I hope Alena continues to get chances, he has enough talent and good attitude that he must be given the chance to learn how to play better. I think Malcom and Boatang should not play any more this season, unless there are no alternatives.