All commander units should be called in from the buildings.. For exaple; ur commander has Tiger call-in, whenever it is unlocked by CP, u can call it in from t4 building..
I remember in the alpha or beta phase of the game. It was like this. But they changed it in the release version, probably after getting some feedbacks..
I doubt that was a reason Relic decided to ditch the idea of callin attached to tier buildings and i never remember relic post regarding to this during alpha/beta.
It was only implemented during alpha stage to close beta, where swappable commander are still not yet been fully implemented yet and they needed feed back on those units. How quickly Relic change them to commander callin ability during the last few beta stage with intention on supporting various Commander (DLC).
As per topic, there are multiple ways of solving it.
IE:
(a) making them very heavy MP investment
(b) limiting them by unit cap (as per discussion)
(c) minimum tier prerequisite (which maybe a bit problematic on balancing tier on both side)
(d) making heavy tanks callin slightly higher in cost and pop cap