You can put five squads of infantry into a bunker, and they can shoot out at passing units. So the effect you get is by mixing and matching infantry types. You could make a strong anti-vehicle bunker by packing them with PLEX soldiers for example, or mix and match to taste. For the Consortium, I find 4 disruptors and one grenadier squad to be an efficient way to clog a choke point
Bunkers are pretty resistant to damage too, so infantry can actually make a pretty large dent in units they ordinarily would be defenseless against. Of course the downside is that they can't move, and as an attacker you have to dig those infantry out of their holes to win the map (assuming a "destroy everything" scenario).