Hi, one of the issues is that DBA is at a different scale to DBMM, so the mechanisms are different, arbeit to produce a similar outcome. Usually in DBMM you have blocks with quite a few spears in them, because of the relative point system, so you can already be both wide _and_ deep allowing you to absorb enemy intrusions (because they usually end up double overlapped) and cassulties.
I think I read an article from Goldsworthy(?) arguing that there was clearly an advantage to deep formations of spears (at least in Greek hoplite warfare), but it was a morale effect. He was definately against the whole "scrum" thing. With DBMM deep spears have an advantage, not from any combat factor (only second rank), but because the follow-up and threat-zone rules force you to continue to fight them. Eventually numbers win.
Lastly, Barker has commented before (Slingshot I think) that he thought the usual neat block diagram you see in books looked nothing like what it looked like on the battlefield. In DBMM, cassulties and recoil/follow-up will see at the end of a few bounds of combat with the line looking very ragged, with some troops pushed deep into enemy lines, or blocks of troops being isolated from the others. This is where the command/control PIP system is stressed because there are way fewer PIPs available than you want. Good players can focus or rallying troops in the best position to make an impact, or better still, have a reserve that can exploit a weakened enemy.
Anyway, just some of the reasons why DBMM is still my rules of choice for ancients.
Oh, and Sp(I) need to be at least 3 deep. Thin ranks of Sp(I) are very vunerable.
Cheers,
JohnG
