I can see why you would think that. It bothered me as well when I first signed up. Honestly the key is in the catchup series. JNC has so many good series that it's easy to find something new to read every month, in addition to all the new ones that are always in pre-pub status. Honestly, once you are caught up on a series, then as long as you keep an eye on the prepubs, then you'll never need to buy any unless you want to.
Discuss J-Novel Club light novels here