Lifehacker has been suggesting using AdWords coupons to get free Dropbox space. http://lifehacker.com/5854955/how-to-max-out-your-dropbox-re.... That's probably why many people are trying this.
I put my OWN money into the account. I also didn't set low bid limits. Actually, my bids were high enough that they came in much lower than I anticipated.
The reasoning was simple: I was paying Google for some benefit. Using a free coupon is simply taking advantage of Google for your own benefit, and that's not fair to Google -- it's absolutely one-sided.