Years ago I was a graphic artist for the LA Times preparing double-truck auto dealer ads. It was a weekly graphic nightmare to squeeze every last ounce of open space out of a two page spread -- 20 new car promotions plus 60 used cars, no problem. I hated doing those ads. It was the same time-frame as when VW was placing one car in the middle of a blank page and eating everyones new-car-sales lunch.
This article at A List Apart states eloquently the reason whitespace is so powerful when creating any graphic element (text, illustration, or a combination of both). The best example of how law firms abuse whitespace is in tombstone ads. Almost 100% of firms using tombstones will line the entire ad space with boxes, inside boxes, inside more boxes in the attempt to list as many transactions as possibly readable. The thinking behind this type of space use is that readers will be terribly impressed with all of the work a firm does. In truth the space is so packed and jumbled to the eye of the reader that nothing but confusion is communicated.
I do like Mark Boulton's point about use of space: Less whitespace = cheap; more whitespace = luxury. Eloquently put Mark.