Website Accessibility is an important thing for serveral reasons. Which of the reasons to be accessible is more important to you is for you to decide.
1) the law was changed and accessibility now includes websites.
2) As the age of web users becomes older it becomes important for your website in both ethical and economical terms to be accessible.
Each year the number of people over 50 viewing websites increases this also means the number of people who are viewing websites when partially sighted is growing. If your website is not accessible then these people will not be able to interact with it.
With Css design, which this php cms model is made with, it is possible for the site to have the writing changed in size and for text by readers so built in accessibility. But we still need to consider that we want it to look good as well as be readable by all. Sometimes designers forget this or if you make your own site you don't know it but if you do not have enough difference between the colour of writing and the colour of the back ground many partially sighted people will not be able to read it.
Dont loose up to 20% of viewers because your website is not accessible. And if your site is not accessible it will not be search engine friendly