Its good for getting your formation into a blob... AI very often ends up with several units attacking single enemy, which causes them to all move to occupy same spot.. it practically makes flanking impossible wiht "formed Attack" units, as they will always shuffle to attack enemy frontally.. more on this in videos below
I reported this in official forum, support section, but so far its practically ignored by CA, and it will be like that, until community wont demand this to be fixed! (thread i opened at official forum:
http://forums.totalwar.com/showthrea...t-Combat-issue )
I know some of you might think there are more pressing issues, but i think this one is the biggest problem of all Rome 2 has, as it completely prevent any faction using such units to use them properly in lines, or using them as flanking force. All you will get is one massive blob with all your units standing on top of each other.