There often discussion about whether or not your pages have to conform to the standard in order for everyone to view them and have robots parse them properly. The answer is no. You can build a page that does not follow the guidelines that will work well for people and robots. You can also build a house that does not follow the building code that would work just fine. But in either case, you should be 100% sure about exactly what you're doing; otherwise what you've built could come crashing down. Following the accessibility guidelines (and the building code for that matter) is not that difficult and it significantly reduces many risks.
To summarize, you have to identify your primary audience to be able to effectively design your website. The next step is Analyzing your traffic to see how well you're catering to that primary audience. Use accessibility guidelines to ensure that your site will be visible to people outside your primary audience as well as web robots. These are all important steps in the long road towards building a successful website.
Keep your audience in mind as they are the most important consideration when it comes to your website.