When I encounter an AT2 I switch to HE and fire at his mantlet and commanders hatch -the other times I use it - stopping a cap when you cant afford not to do any damage i.e. 80% cap or against arty or open top TDs -if you can hit inside the turret of the TD you can do some good damage -Ive 1 shot a couple of Wolverines with HE, but other times just done 40 damage (I might have damaged components or crew),
also finishing tanks with only a few hit points left, HE can be used then
from http://wotguru.com/weak-spot-guide-at-2/
"Unlike a lot of tanks the main weak spots are not on the hull but on the turret(casemate in this case). The commanders hatch and mantlet area around the gun are the two easiest hit."