It's not quite a matter of multiplication. The 48 guys can only attack 48 times. I'd rather have 144 men all with 1 hp with equal stats.

That said, multiple HP means that they can "heal" automatically between battles and need no retraining (ie. lose 2hp per men each battle evenly distributed doesn't show at all).

They can actually wade through the spears and reach hoplites in melee. However, frontal charging phalanxes still does not prove too wise a strategy.