Although Marlin call it a swirl pot I think it would be better described as a surge tank. The problem is that there are no baffles in the fuel tank. So if you tank is say only half full and you pull away quickly the fuel all goes to the back of the tank and the engine is sucking air. Of course if you never plan to pull away quickly or always have a full tank it wont be a problem
For how much it is and the pain of retrofitting I think it is worth fitting from the start.
I beleive some of the early fuel tanks did have baffles so probably wouldn't need a surge tank.
BTW - still having problems mounting my AFM - I am on to the MkIII bracket now. Someone told me that mounting it direct to the engine (which was easy and I had done) is not good as the AFM contains a very fragile thermocouple arrangement that would not last long with the vibration from the engine.