3 x 3 is useful if your CC detachment is spears or halbs. You don't
really want them to fight (and die) you just want to get a flank attach to push CR in your favour. Also, 3x3 allows your main units to be closer together (closer to general etc) and makes it harder for enemy to charge the detachment and leave your unit alone (a sneaky enemy tactic).
If you run FC or Swords you may decide to go with 5x2 as you are anticipating these guys actually killing something on top of the base CR advantage. Those people are wankers like the poster above however and cannot be trusted
Seriously, 3x3 is the safest choice