Not being able to easily take out a tank single handed is frustrating for the individual player but pretty important for overall team play. You're not supposed to run solo and tanks should have an advantage over a single infantry unit. This is especially true for the mobility and size advantage of infantry, where it's not all that difficult to flank and hide from tanks and land every shot successfully. The idea is you work together, and one of your two-three engineer group lands the killing blow, the reward being the end of the tank. But the dumb XP/unlock system makes that reward less satisfying when you get an assist and were aiming for the kill.