I geuss that, being a female, I am more fond of cycle breeding. I believe that females of most any species need a rest between pregnancies. I just put myself into the rat's position; would I want to be continuously bred or nursing without a break? No.
It seems pretty selfish to deny female rats a break, just because we want to pump more babies out of them. Again, this is my opinion. To each their own.
I do have some dry spells, but I am growing up 0.6 right now to supplement this operation.