@xAAAx The reason I don't want Subs to gain a defense is that with their cost they would be viable as defensive fodder. (As we have seen in previous versions of TWW) So then you could stack Subs and use them like transports (except that they inflict first strikes on defense)... except that they would also have the ability to be highly effective at smashing the enemy on offense.
The entire design is meant to make Subs a useful offensive weapon while making them (comparatively) unattractive as cover for a fleet defensively. This is intended to make Sub spamming less attractive. Like stacking 25 Subs off the coast of Germany to defend an otherwise meager fleet.