In the system envisioned, you could do either. You get 15 listings, to do with as you wish. The idea is to allow sellers more choice in how they use their listings, so both your preferences can be accommodated.
There is probably an uptick in usage because we are getting more visits, more visitors, and more new visitors, nearly all of whom are more comfortable with Ebay style classifieds than with "megathreads". The new system will allow you to tailor your listings as you see fit. You can list items individually or as a group, so long as the items grouped together fall under the same category. The limit of 15 listings per seller will prevent an single seller from dominating the entire page of listings.