Agreed, I think the StuG III E could use a larger blast radius, plus get a free barrage ability (like the SU-76). It's supposed to be an Assault Gun, a howitzer on tracks, the least it can do is act like one!
Umm, no, you're confusing it with StuH and brummbar.
III E didn't had any howitzer, only low velocity gun. It was meant to be close infantry support and destroy soft targets and fortifications. Basically things that soviets don't have.
Also(yes, I'm going to say it, because why not, its used as argument in soviet case) you pay no tech costs to get this cheap armor.