Its not exactly a scientific proof, but.... if its of any use....

Yesterday, in my Roman campaign, I was assaulting Carthage. Among others I had a couple of merc phalanx units. And they DID attack.

Phalanx formation, defense mode OFF. double r-click on the enemy, the phalanx gets into formation and charges in perfect ranks. (At least it worked for me....)

What worried me is that they didnt even make an attempt to pursue, despite having the def-mode turned OFF..... might be a "phalanx formation" thing.

Hope this helps....