Glad to see there's so much interest in this, I thought there might
Excellent post Reapy, you pretty much hit it on the head from where I stand.
I would really like to organize some kind of battle or even TDM match and just try out the formation in the "ideal" setting, that is, a formation of soldiers fighting head on with a mob, just to see if it is or isn't effective. There isn't really much use of practicing or drilling unless that is effective, so far the only formation I've actually been able to test properly (without it breaking up) is the "fighting an individual" one, and as others here attest, it is extremely effective, so I'm pretty sure that a large-scale formation would work well also. - Edit: already suggested by Ursca
Reapy your idea of smaller more mobile formations sounds excellent as they could easily maneuver and engage small knots of enemies who would otherwise have enveloped the major group. It's just guesswork here but I think if the three (probably the ideal number) were able to work together when needed and form a large formation for when their foe was not spread out, it would enable them to more effectively deal with multiple situations. Furthermore, they'd likely be able to deal with Crypto's situation with satisfactory results, splitting off one or more of the groups to deal with such problems while always keeping the groups in a position to reform or be able to support each other.
So far the only real benefit I've seen of formations [apart from the "fighting an individual" encounters] is that it holds people together and stops them running off on their own, as a result, the group was very effective. However, this is not formation fighting nor had anything to do with it, this was mob fighting where everyone was present to fight a smaller number of the opposing team.
I think testing the effectiveness of "large" [8 a side or so] formations against one another in "lab" conditions would be the best course of action, that way we can get a base result and know how other tactics are working by comparing them to the "ideal". A few tests with different techniques and weapons would be great:
* Rigid tight formation (staying in a line while fighting and not breaking up).
* Loose formation (manouvering while staying in rough formation, as mentioned before).
* Marin's formations
And with different weapon loadouts such as sword/board, spear/board, mace/board, spear, pike, two hander.
From what I've seen when trying to organize formations, the biggest problem is Hastings Syndrome, people rush to meet the enemy, it's a reflex. Very much like a rout, once one goes they all start to follow. I think the biggest thing to remember about formation fighting is that it's a single unit. You're part of a machine. Think about this, what would happen if your legs and arms all tried to "run" at the same time? Without working as a cohesive whole your body would lie on the ground in convulsions, the same is true of formation fighting.
* The PDF is the same as the OP, but with a purdy background image
And yarg, those be my sketches.