DA staff has been very good at implementing anything we as a group have asked for, if it's easy enough for them to do.
When they don't implment something, my educated guess is that it's not easily done in a short time frame, so it's held off for when/if there's ever a version that's a completely new design.
I once discussed switching from mbox to Maildir with John, and his point to me that converitng existing servers is quite complex.
Jeff