Actually It Depends on How You Define Brittleness
My point-of-view-only, of course, since I'm not Don.
I am not hurl;ing any bricks either, Snake, I enjoy your nitpicks.
Anyone can value the extra hit durability of the AA/BB/CC/DD/EE/ etc. However, it is the actual change of letters which call for various damage effects to occur (Going from F to E, E to D etc).
I think the spirirt of the Starships design is that the letters signify which is better or worse, and that the double letters don't change that. Hence, the value list would be (going from lowest to highest):
A
AA
B
BB
C
CC
D
DD
E
EE
F
FF
In your example, in choosing the worst, it would be the CC not the D.