I registered my drones simply because to use them as part 107 its the law. As a recreational pilot only, I dont see the need really unless you have a bird that's overweight. If you are unsure about the weight of your drone at takeoff, then I...
Personal opinion?
Wherever you are: regardless of the weight of the drone, it is a good idea to have both it, and you registered (as the operator).
You're right, it is cheap and being registered could well save you a load of headaches if and...
You are completely legal, and not required to broadcast RID, nor register yourself as a pilot with the standard battery in the Mini 4 Pro. No legal hassles from anyone. Plus you save $5, and the only thing you give up is a few minutes of flight...