Positives:
-Unit density would be much less.
-Easier for the AI (at least until longer range achieved)
-Limited duration - would only last until catapults and other siege weapons.
-Realism (the degree of which could be argued)
-The ranged/melee component of other units such as Triremes and Galleys, etc. could be supplemented or eliminated. For example, rather than have a ranged and melee version Galleass/Trireme, we could have a ranged support unit on a Trireme that serves the same purpose. This was the reality... until the the true sailing ships and gunpowder were developed, the galley was multipurpose. A Galleas was just a Galley with additional armaments. The nature of the ranged support unit changed from arrows/catapults (if they had them) to gunpowder. In fact, I would argue that even a melee unit should be able to be added to a naval unit (before gunpowder) to form a combined unit. Then the choice would be, for example, to add an archer to a trireme as a ranged support unit, or add a swordsman (perhaps with a marine promotion) to create a ramming/melee type combined unit. Such a naval unit without either a ranged support or melee unit combined "army" would be extremely vulnerable, being in reality only a civilian ship with little combat ability.
Neutral
We do not know how damage to stacked formations is allocated... perhaps the support units might be vulnerable in some way to certain kinds of attacks/units (such as horseman or counterfire)(may require a specific promotion ie targeting or flanking or return fire or somesuch).
Negatives
-Possible gameplay loss (although I think some of the neutral effects may combat this.)
-Too tactical granting the strategic nature of the game.
Others might think of other ideas... or a variety of ways to mitigate the problems from either point of view.