well...many "barbarians" did organize. they used an offensive (or defensive..for that matter) Wedge formation.
like you said: the basic infantrymen didn't have a good grasp of tactics...but their "generals"(for lack of a better word) certainly did...

roman/greek infantry weren't all geniuses like Alexander (i.e.) a lot of them we practically told to hold formation and move up upon the enemy. not knowing teh full plan.

i definatly don't agree with the "almost only warbands". there were a lot of tribes there. with a lot of veterans, every one being different from each other.
because Pezhetairoi were the order of teh day we should just scrap the Hypaspists/ argyraspydai's? or even those bad-ass-Torikatai argyraspydai (sp...)?

i know i'm not na expert. but