I'm not sure I need anything but the 6S after using it for 10 months, but I can see how those with very thick and heavy beards might. In general, I am sold on adjustable razors, whether mechanical or with multiple base plates.
Adjustables and swappable plate razors can really be two separate things.
With my Progress, I change the setting a few times during the shave. Whereas with my 6S, I just choose a plate and run with it.