Should I have a text size widget for accessibility on a charity website?

I'm a developer at a small web agency that works primarily with charities. We tend to put a simple textsize widget on every site (eg. top right on www.embraceme.org). I find myself wondering if that's necessary: if someone has difficulty seeing, then there's a number of tools at the OS/browser level to help them out. If each website reinvents the wheel then that's a new tool the user has to keep relearning (not to mention extra and unnecessary complication on the site). We don't have an accessibility/usability specialist, so would love to hear people's views on this.