Realistically, sidearms are supposed to be a last resort kind of weapon, so I understand why the devs don't really give you much ammo for it. However, a part of me misses pistol play, and its currently impossible given that a heavy vest gives you only 4 mags for your sidearm.
I think there should be a third vest option for say the Marksman class that sacrifices primary mag pouches for sidearm pouches, or an option to add on a MOLLE pouch for a sidearm one at a time, say a point per extra mag.