You'd have to come up with some ad hoc rulings on stuff like archery fire (a mob of archers firing English-style creates an area-effect attack, perhaps?), but it could be a cool way for the PCs to really feel like they turned the tide (or lost the day) in a massive battle, rather than it being all decided beforehand by the GM.
Mind you, I've never been one who was interested in playing out an entire huge battle with thousands of troops on either side ... but it does make for a cool backdrop from time to time. (My Star Hero campaign's climactic adventure was an epic starship battle in orbit over Earth, with the PCs' role being to blast their way into a lunar base to defuse an orbit-destabilizing bomb. Think the opening sequence of Revenge of the Sith and you won't be too far off.)