Not that much, you say, but it might contribute a little bit to provide a more pleasant experience. So does the screen quality. Its all a big sum which adds up to an end experience. So does hinting. Rest assured the hinting on Kindle is optimized for monochrome screen, and it probably has a proprietary font as well. Heck, for the first Kindle one main complaint is the contrast, and by default Kindle will not show non-English characters like Japanese well. In case of English and other Western languages hinting is just less important than where the pixel density is already always good, like on LCD screens.

At the moment they are big enough to read it still matters what kind of font you use! Go use Netscape Navigator 4 on SunOS or IRIX and tell me you find those default fonts readable. It is simply a horrible experience no matter how big or small you put the default fonts. If I compare that to iPhoneOS and MacOSX default fonts these provide a much better experience, in different screen sizes and on different screens.

Why? Eye sight changes throughout life, and it is not uncommon that middle age people have to buy reading glasses or get multi focal contact lenses. I see it here all the time, although it used to be part of the standard insurance which it isn't anymore since 1 january 2009.