Planes were very effective sub killers in reality. However, reality suffers a bit when translated into an abstract board game. Sometimes the designers have to “fudge” one aspect of a reality-based game in order to deal with the limitations of another aspect of it. Hopefully, in the end, the two balance each other out to create a whole that abstractly mirrors reality even though the individual parts may not. The interaction between subs, destroyers and planes is a perfect example of this.
The range of air units and the size of the oceans in the game make it very easy for air units to find and eliminate subs, if air units are capable of hitting them on their own. This gives subs no place to hide and makes them “sitting ducks” for air attacks, as was demonstrated in the Revised edition. This simply doesn’t reflect the realities of anti-submarine warfare as it occured in World War II, at least until long-range aircraft were developed later in the war. Until that time, subs were very safe in the middle areas of the oceans, as planes didn’t have the range to hunt them effectively there. Most were sunk only when they were caught in the act of raiding shipping.
Adding the rule that destroyers are required as “spotters” for air attacks against subs represents the concerted effort needed to hunt and attack subs hiding out between raids. This gives subs more longevity and makes them more the fearsome foes that they actually were in the early to middle days of the war.
It also promotes the purchase and maintenance of more well-rounded fleets, as destroyers are necessary to guard against the threat of subs. This reflects the reality that subs were a constant threat to military shipping as well, and that no convoy would travel without destroyer escorts because of that threat. At the same time, it keeps them from being used as cheap “cannon fodder” in naval battles, as they were most often not used extensively in fleet operations, but rather as harrassing hunters where their unique properties were best utilized.
From an economic standpoint, the necessity of buying destroyers for protection against subs also reflects the economic losses sustained by raids against merchant shipping by submarines.
All of these points, taken together, allow the game to abstractly represent the economic and military threat posed by submarines in World War II. This makes subs a useful and strategic purchase in the game. I hope this sufficiently answers your concerns.