Your core should be balanced for everything, why would someone not call in their core if they attacking/defending instead of me/r mode or vice versa?
I'll give you an example.
First off most people have multiple starting call ins. I always do on my axis for the most part, because it's never a bad idea to have a P4 in your starting call in.
In my armor company though, I have a single starting call in, but I also have a call in that i bring on mid game of engs all with mines. but as soon as the round start, most people call their stuff in ASAP, because that extra second can get you a building position you need. well you call on your core, then you go, oh shit... we are defending... too late now, my cores on the map.
i agree its a back burner idea, but it woudl be great ot have implemented at some point.