In reality, ACC means ZILCH in actual gameplay. If you edit a player to have zero ACC he will act and move exactly the same as if he had 99 ACC. I goofed around one time and created a WR with 99 in everything but ACC, making ACC zero, and his overall rating was somewhere from 80 to 85. I know that I created a WR with 93 SPD, 74 STR, 99 AGL, zero ACC, 78 AWR, 90 CTH and 80 BTK once to see how it would turn out and his overall is a 77. I created a running back once with 60 SPD, 60 STR and 99 in everything else and his overall was a 99. The ACC rating is a huge modifier in the OVR rating as you can see, moreso than speed. I guess you are wondering what ACC does? It affects yards after catch, yards per carry, kick and punt return average, and the yards per carry of a QB if the offensive modifier is set to option run. If it is set to spread it is rather generic. I know ACC is taken into account on the depth chart when looking at return men, as the more highly rated returners have not only higher speed, but also a higher ACC.
All of the above is only for SIMMED games. By the way, the yards per carry stat hinges mainly on ACC and BTK. That back I edited to have 60 SPD and STR, with 99 in everything else? His yards per carry was 6.8 for over 1200 yards during a simmed season. Everyone on this board knows that a RB like that would get a broken tackle here and there, but he would be lucky to get 2 yards a carry against a human opponent.
I just wanted to clue you in. During a simmed seasion the stats I put up would get about 4.5 yards a carry simmed or in game.
As far as defense in a simmed game/season, I have no idea what affect it has. In game, same as offense. None.