The extra men behind the two front ranks would keep up the fight against the enemy front ranker long enough for their battleline to win elsewhere. So everyone in the battleline stood a chance of getting involved in the battle.
Hmmm…the man in the 7th rank would have to watch 6 of the blokes in front of
him being killed (that’s 87% casualties) before he got the chance to strike a blow…
…I’d have been off and running long before that!
But I do agree that the best fighters were at the front.
After all, there’s no point in having your best men behind.
I just question the notion that they stayed there
for the whole battle,
no matter how tired and exhausted they became.
Fighting makes men tired…
Tired men don’t fight as well as fresh men…
And the longer the fight goes on, the more tired they’ll become.
If not relieved by the fresh men standing waiting behind them,
eventually these excellent but totally exhausted fighters will be
facing not so good but fit and fresh fighters.
Who has the advantage then?
And to have fresh fighters to relieve the tired men in the front,
they need to be formed-up in multiple ranks.
Plus relying on Livy alone is a bit like reconstructing the history
of the Second World War by watching “Kelly’s Heroes”!
Still, all ancient historians have some sort of bias (even Polybius).
So much cross-referencing from multiple sources and reading
between the lines is required to try and find out the real truth.