It seems that many people are whining that BtC comms means they will be forced to play X character or waste comms until they manage to hit the magical 4200 number.
Why not make a trade-in option for 2100 that gives you half a heart? Or better yet, 1050 for 1/4 of a heart? This would lessen the sting of BtC as you could trade them in sooner, thereby lessening the "waste." This also allows the devs to keep them BtC for any future plans they may have.
As a secondary benefit of this system, it prevents the comms from "spilling out" of your bags due to the large number of them.