Since Diamonds get C+ automatically immediately after booking, no matter how long in advance you purchase your ticket. Better to throw the cost differential at a higher Main Cabin fare class to improve your F upgrade chances (assuming domestic) — say an M fare over an L fare — than to spend it on C+.

No earthly reason for a DM to actually spend money on C+ — at least today. If DL really wants to make C+ a real class they should rank it above MC in upgrade priority.