Understanding why it is necessary to make web accessible websites I believe is the first step of getting more designers and developers making the web a more accessible place.
Ian Hamilton, the author of the article, also goes into other aspects such as cognitive impairment and associated motor and medical conditions like RSI, hearing impairment and visual impairments. All of which can put a users web experience at a disadvantage. I remember working for a director many years ago and asking for design approvals when finding out that they were colour blind.
It made a few designs a little difficult to communicate and get approved. It is a great and well piece together introduction and well worth a read for anyone that is either already into accessibility or just getting started and understanding it.