ost may have a couple of high-pen options in their arsenal, but i fail to see how that makes mark target redundant. the -25% armor debuff is actually a 33% better chance to pen and makes a huge difference in many tank vs tank matchups.
for example, the Pz.IV with its 110 far pen has a chance of about 69% to penetrate the armor of a regular sherman at max range. with mark target that goes up to 92%. and if you calculate the probability of killing that sherman with the first 4 consecutive shots (excluding acc for simplicity), your looking at a 23% chance to do so without mark target versus 70% with the debuff. and that's even before factoring in the accuracy increase, which is almost always helpful.
not to mention there are also some high-armor tanks in the allied roster which can bounce even the occasional pak or panther shell... hence, i'd say the ability is far from useless.
that requires 2 p4s to work.. 1 to mark target stuff and another to benefit off of it. Easier to make 2 regular p4s no?