Last few cruises my bill at end of cruise has been ZERO. In fact, we have had to use the spa, boutique, and very expensive whisky just to use up shipboard credit.
Marc.. u forgot to mention you sailed on Regent (a luxury cruise line that is all inclusive), so your initial up front costs are not comparable to main stream. Though you are right to point out the all inclusive nature, that's quite a small segment of the cruise market.
However, even in that case, I bet you had other costs; transportation to from the pier for example. Plus any tips you may have given the staff, even though gratuities are included in your cruise fare. Plus tipping guides. Plus any shopping you may have done in ports of call. Those are all costs that do add to your total cost.
If you HAD too spend onboard credit just to use it up, then you also obviously made purchases you normally wouldn't have. Nice in one way; not so nice in another, as one way or another you do pay for the shipboard credits.