Sure I can see the point, but it does essentially double up on your reloading and ammunition requirements. The batteries themselves are reusable, but they aren't cheap and they require time to recharge.
As for using more than 30 shots. I can see it happening easily in a longer-duration mission, say in a situation where you're in a feral city like Chicago or some african hell hole. A SnD mission where you're sweeping out animalistic/pest creatures like feral ghouls. A wilderness adventure where you can't easily find a place to recharge so you have to bring all your ammunition and battery packs with you.
They're certainly not the standard, but then again running into a situation where you want err need to use an assault cannon isn't either. Well not unless you're in an overblown neon pink spiky mohawk campaign, in which case hells yes you can blow through more than 30 shots.