In the main bathroom extractor fans have very thin wire in the winding and to protect the fan from burning out due to being stalled would need a fuse in the milliamp range. So in real terms the fuse only really protects the wiring, so be it 6A or 3A is really no difference the motor will still burn out first.
Clearly all cable needs some current limiting device, so one would not connect to the 32A ring final without some thing to reduce the current before the circuit opens.
As to the permanent line and switched line it is likely that the switched line is simply a trigger, but manufacturers don't tend to tell you if that is the case or not, so both have to be treated the same.
The big question is if the fan is required under building regulations or simply that the air needs changing? The building regulations talk about natural light and ability to open windows and often a simple trigger when the light is turned on is not enough, double pole switches, pneumatic timed switches, and PIR's may be required to comply with building regulations, but if fan is not required then a simple light trigger is enough.
However having a fuse has a second use, should the fan become faulty then you want to be able to isolate fan so light still works, in this house the fan isolator is always turned off, fan was fitted to get completion certificate and once issued never used again.