Sticking with the soccer analogy...each player should be talented in their individual zone and skill set, yes; but having a defender who can overlap the midfielder and go to goal is quite an advantage. Midfielders usually play both ways effectively. Referring to the MLS, if you watched the Sounders play up your way you'd notice that Leo Gonzalez and James Riley frequently advanced up the field into the attack when the opportunity was there. (okay...so 27 years of soccer playing/coaching/watching is revealed here).
I agree that there will be three 1v1 matches going on, but if one gets lopsided, changing thirds may be the only way to rescue your alliance. Also handy if you have a dead bot, flipped bot, or a no-show to a seeding match. Ever get paired with a team who forgot a charged battery, lost a motor, or ???
If your alliance has cleared most of the balls into the attacking 3rd it would be beneficial to advance to the midfield to aid in the movement of balls to the attacking 3rd. (and if you are that dominant you'd better give the other alliance a chance to score anyhow as per 9.3.x).