Well, good things can come out of this. If your boss sees that you cover for others frequently, you may be more likely to get more hours and more trust from him. I do it all the time and I consistently get more hours than anyone there as well as have an easier time when I want to request off a day in advance. I remember one week when a co worker went on vacation I got almost all his hours which got me a 47 hour work week. (dat' overtime)

Your co-worker is a dick though. Use his laziness to your advantage. Well, unless your boss is a dick and doesn't care. Which is different from being a cheapskate. If you work for a big corporate entity, most likely there is a pretty low cap on the total amount of hours they can give out so the company can save on payroll.