Destroyers are very powerful, but very expensive ships. They are flagships, the créme da la créme of the fleet. The mothers of space faring war machine... and the only ships in FS that can carry fighters and bombers. But isn't that kinda stupid? I mean, if a fleet loses its destroyer(s) it hasn't any fighter and bomber support left at all. E.g. Koths invasion fleet in EP: After they have lost the Normandy and the Repulse, the remaining forces were on their own - and had to surrender.
I think [V] should have added one smaller carrier type vessel to any species.