Automated accessibility tools are a piece of software which can test a web page, or even an entire website, for accessibility and some companies rely only on these tools to gauge for example compliance with U.S Section 508, the British Disability Discrimination Act or laws of other countries worldwide.
Using the tools isn't bad, but using just those tools with no manual check is bad. That is a mistake!
About manual evaluation, the Web Accessibility Initiative (WAI) of the World Wide Web Consortium (W3C) says:
"Examine page selection using relevant checkpoints from the Checklist of Checkpoints for Web Content Accessibility Guidelines 1.0. Note relevant can mean: checkpoints that cannot be evaluated by automatic or semiautomatic tools; checkpoints that actually apply to the site (e.g. if site contains no audio content, skip those); and, as a minimum, those checkpoints that apply to the level of conformance you are evaluating".
Despite the best efforts of the W3, HTML/XHTML validation and their Web Content Accessibility Guidelines only go so far as to technically assist accessibility devices grab a websites content and make it usable.