Well, there's another reason it's useful to have elements in the same unit. When that units targets an enemy unit in battle, the members of a unit all target elements in the same target unit. So heavy artillery in a corps will fire on the same unit the infantry, tanks, etc. do. Helps to concentrate firepower against the same target.
Regarding tanks, again it depends on how you look at it. Having the tanks reduce entrenchment on the target the infantry are going after could indeed be helpful.