The advantage is code overview, more logical division of responsibilities and doesn't waste unused resources. This stuff with skills has always bugged me in c-script. What does an entity object do with an inventory value of the player? That's not making sense. Instead, the player should keep track of the fishes (entities), and manage their index position in the array, because the player has the fishes. The fishes shouldn't care whether or not they're in an array. They should just do their own thing.

So you lay the responsibility of the fish management to the swarm object, and the fish behaviour to the fish itself. The player tells the fish where to swim, and the fish will do the rest. You probably don't want to make an entity object for the swarm just to have the skills available. Instead, you can make your own structure with only the data you need. An entity has more data than just the skills that are then unused. Would be confusing and wastes resources.

I hope it's a bit clear.
After re-reading my first post I have doubts that everyone can understand it .

Regards,
Joozey