This question is asked, in many ways, in nearly every presentation I conduct. I have yet to find any documentation that indicates providing server-side add-ons, like SiteCues from AISquared/VFO / is a mandatory requirement for accessibility compliance. Yet, many web developers are seemingly backed into a corner by individuals, and end-users, telling them they must do it.
The Web AIM, 508 Checklist provides guidance that suggests if a product is provided to allow for the adjustment of colors on a web site, the tool allow for a variety of different colors and contrast levels, but does not link to anything that states this is a must have. The Section 508 Quick Reference Guide provides details on how to make sure that this the requirement is met, if the tool is provided, but again does not specify that a such a tool must be provided in order to meet compliance standards. With this said, however, many businesses, organizations or states may have more extensive guidance regarding the addition of such tools to a website.
I think of the addition of such tools to a web site much like I think of the motorized shopping carts provided at some big box stores. You may notice the motorized carts at stores like Wal-Mart, but not in your local second-hand store. This is because they are not an absolute must-have to meet access laws for people with mobility impairments, but they are a convenient nicety for customers who may need them. If an establishment does provide motorized carts they are required to maintain them, but if they do not provide them they are not in the wrong.
As a developer, before dismissing such tools as a must-have, check state laws and organizational policies. I also encourage you to consider the demographic of people who may visit your site, and whether they can afford, or use, the, expensive and usually comprehensive, assistive technology, or even the updated computers and browsers with built-in accessibility tools or add-ons for accessing the web. As a consumer, who may benefit from the convenience of having such tools available on a website, rather than configuring, and re-configuring, your own computer and/or browser please be considerate of the developers, and understand the state laws, and organizational policies, that they must work with-in. the addition may not be a requirement that you are entitled to, but if you ask nicely they may consider the addition.