It's important to mention that an immediate switch from proprietary software development to Open Source development would not be a wise decision. The changes in concept are massive, and prematurely adopting this model would likely impact negatively on the economy in the short term. Rather, gradual adoption to Open Source development is recommended.
Like Richard Stallman (1992) I agree that a switch from proprietary software development to an Open Source model could easily end up with a smaller market overall, but this market would be more efficient because of less duplicated effort. After all, bigger isn't always better.
Ethically, Open Source development is a nicer fit than proprietary. It lends itself to many popular ethical systems. One good example of this is Utilitarianism which states that the appropriate course of action is the one that gives the most benefit to the most individuals. Given two applications, one open one closed, there is no question that the open one is of more use to society as a whole. The only question is whether the open application could exist. As I have defended, Open Source can easily generate revenue. If Open Source was more widely used as a development model I have no doubt that the quality of software would be greater.
Developers programming under Open Source can be confident that what they are producing will contribute positively for the greatest number of society. Also, as was previously mentioned, the Open Source community is very active. All members, whether programming, working on documentation, or simply using an application, are seen as contributing to the greater good. It's a breath of fresh air, and a fun place to be.