How often are you really in that kind of a pinch though? Unless you're doing something seriously wrong like blowing all your heavy on trash mobs before a big boss fight, 99% of the time, you should have enough to help you kill whatever needs to be killed. Even in the raids, it's not a big deal. You do an ammo run on one of the stages, wipe, then start with full heavy essentially. Not as convenient as ammo synths but, you still get what you need.