The ability to change font size can have an enormous impact on accessibility. Pinch and zoom is wonderful for this on iOS, but it introduces another problem. Zoom to much and you now have to scroll sideways as well as down to consume content.
There is however, a little workaround. You can increase and decrease the font size on a per site basis in Safari. This is done through a bookmark, adding two bookmarks one for increase and one for decrease. You can manually set the appropriate font size. Reloading the website will return the font to its original size.