The Business Value of Accessible Software

Good accessibility is good business.

• It provides access to a market of customers that otherwise cannot spend money on your product.
• It provides a competitive advantage over companies that are not accessible - Dell Corporate has lost bids to more accessible competitors in the government sector, and we'd rather be the company others lose bids to.
• It frequently provides higher quality than inaccessible products -- products that are thought through from an accessibility lens are often easier to use for non-disabled users, or provide features that otherwise may not be thought about. (For example, while Closed Captioning was developed specifically for Deaf audiences, it's a service that almost everyone has used at some point, whether at a crowded bar or in a room with a sleeping baby.)
• It ensures continued use of the product to customers that transition from non-disabled to disabled. (Keeping a customer is just as important as gaining a customer.)
• It lowers legal risk of civil rights and access lawsuits.

Further reading:

• Why Designing for Accessibility is Simply Good Business by Digital Design Standards
• The Business Value of Integrating Accessible Technology into Business Organizations by Microsoft
• Why accessibility is good for business (according to my mechanic) by Nicholas Steenhout at Simply Accessible
• The disabled community is the world's third-largest economic power by Christina Mallon at Quartz.