Just because a system has an extra filter and a pump doesn't make it an effective polishing system. Some key features that do help are:
- fuel pickup from the very bottom of the tank. This is where water and other guck will settle, and if you system can't pick it up then it's doing no good. Dip tube pickups are typically designed to have exactly the opposite characteristics and intentionally pick up from a point above the guck. Most of the Nordhavn I have seen work this was and as such are advertised as transfer systems, not polishing systems.
- a fuel return location and fuel flow rate that will actually cause some agitation in the tank. This will stir up settled gunk so it can be sucked through the polishing filter. Most builtin polishers do not accomplish this as all since they use the normal top of tank return port. But this capability is really only important to rectify a long accumulation of gunk, and if you have such a situation, a polishing service should have the necessary equipment with return hoses that can be directed to different areas in the tank.
Because of all this, I seriously doubt that most boat polishing systems provide any real benefit that is not also provided by your main filters and the engine constantly circulating fuel. That will remove suspended impurities just as well as a polisher using the same pickup and return ports on the tank.
I think the best defense against bad fuel is to have some sort of fuel segregation policy where possible given the boats tankage. Purchased fuel goes in one place, and fuel to burn goes someplace else, and never the two shall mix - except via transfer through a filter and water separator. You can call it a day tank, but I think there are other ways to accomplish the same thing without a specific day tank. But it's all highly dependent on your boats configuration.
But segregating purchased fuel and burn fuel doesn't absolve you of the need to monitor and keep the purchased fuel tank clean, and so brings us right back where we started. But al least bit keeps the crap away from the engine, which I think is priority one!