Evolved if we relinquish C, we are only fighting for 3 flags, the enemy already has one. If they manage to get just another one it's a stalemate and the bleed is defined by kills. What is the easier flag to hold and kill attackers? C!
Remember if we are consistently holding 3 out of 4 flags we are not having a close game.
You only need 2 good players to defend C. One on the second floor of the building can easily watch out both stairs. Your best bet to take out a guy camped there are explosives (grenades, M320 or RPGs), but you're more open to then than he is because of heigh advantage. Another guy can easily watch out to tunnel leading to the russian spawn. You'll still have 10 guys going around capturing other flags. I'd say it's a good trade off.
You'll only need more than 2 guys to defend it if the enemy is committing more than 2 people to attacking it thus leaving the other flags open as well. See were I'm getting at?
Your argument about good teams holding C to mow down lemmings is fine for easy pub games, but on competitive games there are no lemmings to be mowed.
A flag is open to anyone considering how easily tanks can get to it, that's why I think it's the least important. Getting to D from B is not that easy. There's only one way (the street) and D have the heigh advantage. It's easier to hold D if you have C than hold D if you have B IMO.
From C you can get to D even from out of bounds. So you can go from the main street, from the out of bounds corridor from the russian spawn. If you go from the street you can get there from C by the tunnel leading to the russian spawn or by the middle corridor that gives access to A as well. There are at least 3 entry points, 2 of then converging on the same place.
