[Taqwus]

Order of reinforcements then becomes an issue... and whether to allow individual squads, or only when there's full room for a commander -and- all his squads, and whether they should ever refuse to come in (morale check, battlefield enchantments, et al).

Unless players could determine the ordering, there'd be complaints when scripts and formations no longer made sense. Might be interesting, and there's logic behind it (although there are obvious questions as to whether the reserves should really be immune to Wrathful Skies, Heat from Hell, the opponent's reserves et al while off-screen), but it'd require some UI overhaul.