Kev, it uses the same gun as the M4A3(75), and thats at least usable up close or on someone's flanks (its mounted on a damn M3 for christ's sake. You can out run any tank, so I don't think flanking will be a problem). You can kill a T-34/76 in one shot at at least 1000yds. I know the T-34 has more than 81mm of armor (factoring in the slope).
Also, HE rounds WERE used, so you could get 59HE round to the town as fast as an M3 can get there. It would be a faster, more powerful LVTA4 when carrying them. Personally, I see no reason for excluding it, as it will add something that will improve the game, which is always good.
But honestly, I think this would be a good test for tank destroyers. And since it would require a minimal ammount of change, I figure that it wouldn't be too much to ask for.