I think we all know 3 or 4 pts cohesion difference won't make that much of a difference but 30 or so will as shown in Ben's other thread.
On Cohesion. - Front Office Football Central
Some people make the mistake that 100 pass cohesion vs 80 secondary cohesion means the pass attack has +20 cohesion when it passes. But to get a 100 pass cohesion may mean your players only need 4 years average on the team, to get 100 secondary cohesion may mean you need 8 years average on the team. Therefore the secondary cohesion in this instance is way higher than the passing cohesion giving them a big advantage according to Ben's other thread.
Obviously Jim may have coded the game to give each unit with 100 cohesion the same bonus but that would make no sense.