Sometimes you have 12 to spend on fleet. Would you rather have a destroyer or a cruiser?
Cruisers are the most cost-efficient unit for bombarding.
Sometimes you have limited production, so in those cases higher quality units may be appropriate. For example, you are building at South Africa and you want 2 ground units and a boat. Or what about when your factory is damaged? Then a cruiser basically costs 13 and destroyer costs 9, so the percentage increase for a cruiser is not as great. (4/9 instead of 50%)
Yes, they are probably the least purchased unit. Their best application is narrow. 95% of the time they are not the best buy.
What about when you have established naval dominance in an area and your opponent cannot possibly airstrike you? Destroyers are then worthless. Cruisers are still very useful, and project a threat. Actually, even when you don’t yet have dominance, the cruisers project a threat on the coastlines in addition to naval offense and defense, and destroyers do not.
Bombardment is immune to AA fire and is zero risk.
There are more arguments for cruisers, but they’ve all been stated before.