Frui cages with no fruit?t

Seen plenty of fruit cages shoot up over the last few months in our local allotment, you know the type wooden with the netting all around it. However everyone seems to be growing brassica's etc in them which I don't understand as these would be far too big to move so does this mean they do not follow the rotation rule?

Come to think of it quite a few at my allotment just grow spuds year on year so rotation is not a thought. Does this mean that if you look after the ground well enough and feed it the right food rotation is not as vital?