I agree that it would be harder for tanks, as they couldn't see infantry as good anymore. But what the hell, that's how it was right? If you want to be a successfull tanker you gotta watch your perimeter, if you don't, you deserve the death.
At the moment when driving a tank it still feels more like beiing in some kind of mech suit with perfect view and total control of all functions, i agree it is practical but it certainly does not feel like beeing in loud, cramped vehicle with tons of steel around you.
Adding vision slits wouldn't be too much work anyway, as they are ingame already! And the immersive bonus it gives counterweights the disadvantages like infantry attacks IMHO.