It'd be like a bad Monty Python skit...
"I killed two buildings with my grenades, now I have enough perks for a Thompson submachine gun!! Ut-oh, Hurricane strafing the town...RUN AWAY! RUN AWAY"
Adding playable infantry requires taking the game to the micro level, whereas we're currently in the macro phase...ten troops to take a base, town fully down, etc.
What would happen, is that the ground scenery would immediately become insufficient, players would demand more detailed buildings, roads, foxholes, bunkers, etc... It would mean an entirely new scenery set, and chances are it would make the program itself a memory hog and stress the existing format of the game.
A compromise of sorts would be to improve the infantry model and controls, and perhaps reduce the number of troops required to take a base if done with live players. How would the perks be issued for players capturing as infantry? A new set of perk weapons for the infantryman, perhaps anti-tank, flamethrowers, medics, or vehicle repairs (billed as engineers).
So maybe we can take a base with 5 live troopers after the town is down. That scenario can lend it self to troop insertions, lots of guys running in and out of buildings to avoid detection, and infantry defense as well.
From a programming point of view, I don't see it as a major obstacle, infantry players are treated the same way tanks and planes are in code, the target boxes are smaller, weapons codes are basically the same, etc.
The question is, would it be worth it for HTC to implement something like this? Would enough players use it, considering that's not what Aces High is based upon?