Depending on the bio load. How many/size fish? How much water in the entire system?
A high load will increase organics leading to algeal blooms, poor water clarity and the fish could become stressed.
The need is subjective. I ran a 125 fish only and the amount of skimmate produced was amazing. Even a high load I didn't have much in the way of algea problems.
Your tank, your choice.
My advice, get a skimmer.
Good Luck!