The Stug seems designed to be just medium AT to be used in conjunction with shreck-PGs (in their fancy new halftrack, of course) until you can get a Tiger.
This is simply not true. The Stug E does not function as any sort of AT whatsoever. Its lackluster penetration combined with only 80 damage and no turret means that you're not going to be killing anything armored any time soon. It probably is less effective than an upgunned scout car at killing light vehicles. Generally in those situations you speak of, the panzergrenadiers probably have it handled by themselves.
For those players complaining about its effectiveness, I see why this tank would be regarded as redundant since the assault grenadiers and panzergrenadier halftrack call ins are more than enough anti-infantry for most games. However, the Stug is not vulnerable to sniper fire (or any small arms for that matter) and are effective at stopping shock troops since the shells obviously bypass 2.25 armor (for those situations where you want to rush a Panther, which are really effective against Su-85 spam).
I would not recommend its use versus players who pick guards, for obvious reasons.