Easy answer:
Without the added penalty, you could drive up your loaded APC to the MG nest, unload the engineers, blast the MG with flame/explosives/SMGs, reenter the APC and drive away..
Hardly realistic, and doesn't make much sense gameplay-wise, IMHO.
Now, you can't dismount under enemy (or friendly
) fire ... what was a big NO-NO when I was a Panzergrenadier anyway, as it inevitably leads to high casualties.
The APC has to retreat into cover. That has the added advantage that the enemy does not exactly now where you unload the inf, and what kind of inf it is.