Yes, it occupies a slot, but if there are no other warriors and 3-4 empty slots...

I wouldn't recommend that. When besieging a castle, you need to fill these slots with meat shields.
Usually, towers shoot at goblins, so you can take them. I always have dogs in my army. Also, orcs, as they are archers. Ogres, birds, and hippos are also in the army. And the last slot is for cyclops (but I don't always take them, as they require a lot of crystals to build). So, there are almost never empty slots.
By the way, in The Succession Wars, you can split up warriors directly in battle. It's convenient, and such problems don't arise.