personas can save you from yourself

You may end up paying for lots of design considerations and affordances that you may never need, solely based on the internal goals of the design firm you choose.  

For example, it takes a significant amount of time to debug certain CSS and JavaScript code, not to mention the introduction of hacks and workarounds to overcome browser compatibility issues.  Typically, you approve a design that addresses the specific needs of your ideal customers.  Once you approve the design concept, often times the target audience takes a sharp detour and your developers spend time building out your design for people that will never see your website or purchase your product.  

Your user research didn't include personas for people with disabilities.  However, your developer is spending lots of billable hours making sure that individuals with such conditions can comfortably navigate the information on your website.  If accessibility is part of the business model of the design firm that you choose, then you may end up paying extra for work that addresses the needs of people that may never visit your website simply because the interactive agency that is designing site has a reputation to uphold in the accessibility community and will force this upon you in the build process.  A better use of that billable time could be put towards designing email templates for branded newsletters and sales collateral.

I view accessibility much the same way I view the mobile web.  An accessibility strategy should not be to develop a site that "degrades" to fit people with disabilities the same way that a mobile strategy should not be to simply degrade the desktop site to be navigable on a handset. I speak of accessibility not in technical terms, but in terms of assistive technology.  People with disabilities should be given the same level of care and user experience as any other user.  A separate mobile site built from the ground up to fit the context of use naturally delivers a better browsing  experience to the end user.  A separate website designed from the ground to present information in a format that is most conducive to people with disabilities will naturally perform better and make for a better user experience.  

The advent of the spoken web seeks to bridge the worlds and draw a focus to the format of the content itself, packaging it in such a way that delivers a consistent experience to all users regardless of ability by speaking, listening, or reading.