The point of the tank's secondary gunner in Battlefield 3 is to protect the tank against infantry and lightly armoured targets like helicopters and Jihadi Jeeps. If the support class is capable of getting close enough to plant C4 on the tank, then the gunner is not doing his job, or the driver has put the tank in a situation where the gunner can't do his job.
In real life, it would be suicide to get that close to a main battle tank like the Challenger 2 or T-90, especially considering that you need specialised anti-tank munitions just to penetrate their armour - there's been at least one case where an up-armoured Challenger 2 MBT has literally rolled over a massive IED with no real damage to the chassis.
Besides, if one uses the proper setup, supports rushing it with detpacks are of little concern. Reactive armour minimises the effectiveness of C4, and the gunner or CITV operator using motion sensor allows the driver to see on the map where close infantry are in relation to the tank.