There are other reasons for TV ads. One is to inform consumers about a product or service that they didn't know existed. Viagra and Swiffer come to mind. There is a difference between why Viagra bought ads (and now no longer does), and Charmin ads.
I'd be willing to bet that Dropbox runs an ad in the 2012 superbowl. It only costs $3M, and they are a new product category that has mass appeal. They need to lock down their brand as the goto place for the service. Much like swiffer is now the brand for disposable mops, dropbox need to be the default brand for "your files available on all your computers and devices".