There are 2 blunders here that I can see: 1) short duration of coupon, and 2) inaccurate details (expiration date).
These coupons are good for those who were already able and willing to buy a game from them, meaning that Stardock actually lost out on 20% revenue. Giving coupons to these people usually don't benefit the company monetarily, except making a loyal customer base happier than they already are.
The more usualy point of a coupon (for those that don't understand marketing) is to create interest in a brand. You "lure" customers to your store/website by offering a discount. It's usually for customers who normally would not have bought anything, but now will actually buy. It is in those circumstances that coupons are beneficial, both in terms of revenue and new customer interest. A short expiration date is good for the first group, but are bad for the latter group (most will likely not buy anything at all, until maybe the next promo).
On another subject, the generally accepted standard is when it says, "Expires: June 15, 2009", it means that for the entire day of June 15, 2009, it is still usable. Every coupon I've ever seen operates on this standard, and more often than I should, I've used coupons on the "expires" date. They've always been valid. For example, if you credit card says, "Expires: 04/09", it's usually AT LEAST on the first day of April, depending on company policy. It doesn't mean it expires by the end of March 31.
Both of these things make me, as a typical customer, think that Stardock doesn't really care for my money, as someone stated already.