I'm no expert but having consulted vets and fellow ferret owners, the general train of throught Frank is basically if the jills are not taken out of season, the persistent oestrus produces excessive hormone production & oestradiol, which is bone marrow suppression, and a high chance of aplastic anemia.
All the above leads to loss of condition and high chance of infection etc if left in season without using any of the previously mention methods of bringing them out.
There are pro's and con's for each method but then it's down to the owners to make a responsible decision on which route to take.
Hope that explains a little ?
Cheers
Rb