Shoot, I don't have a basement now. I haven't HAD a basement since I was about 13 years old.
If you got more stuff than your house/garage can hold - sell some stuff.
Why? I like my stuff. I don't want to sell my stuff. I want a basement not only to store the Christmas decorations, home repair equipment, and some books, but also to have a workshop, a little art studio in an area where the smell won't bother everybody else, and a laundry room big enough to sort laundry and open up an ironing board while I watch TV. I want a place to hang my saddles and bridles without risk that they'll get cooked by the heat of a garage in summer. I want a place where the kids can have their friends over to watch dvds and shoot each other with pellet guns, and I won't get hysterical because they're leaving potato chip crumbs on the Persian rug in the living room or wrestling around near antiques.
In the case of these people, they were trying to run a business out of their home and they may have wanted kids at some point, so having a lot of bedrooms makes sense.