Fact is Barry needs a younger player with legs to help him out, and we can only have two in there, imagine how fast we'd get the ball back and close down in midfield if you have two tigers like McCarthy and Besic working as a unit.
Martinez will never drop Barry, ever, he's his favorite player in the squad.
Dropping Barry and Kone, bringing in Besic and replacing Kone with either Cleverley or Lennon/Mirallas would transform this team onto a whole new level.
A midfield defensive two from Cleverley/Besic/McCarthy would be a step forward for sure, a balanced team with fierce ball winning abilities in the middle and great attacking flair upfront.
Never going to happen though, Martinez has his favorites and won't budge for the good of the team.