Yeah I know what you mean and agree somewhat, about keeping it the same size, and keeping text legible. But I have found that its possible to use relative sizes and for it still to be legible at different resolutions and such (I am not sure how this compares to using different DPI's??)
Also, its possible for the user to change the font size. In Firefox its just a case of pressing Ctrl++ or Ctrl+-. This does not "break" the way my page looks (this does in fact damage my arguement about things scaling correctly, but let's ignore that)
I think overall, relative sizes are better. If you change resolution, things scale accordingly. If you have the chance, look at
my website and tell me if it works ok with different DPI's. I have used ems for the sizes of the text, cos W3C told me to
Your point about people with vision problems is true, which is why all of my images have alt tags (needed to make XHTML 1.1 validate). I also have given them title tags to make tooltips (title is the correct way to make tooltips, NOT alt tags!!)
I guess it all depends on the person. After all, although I think a bit about it, I do not cater too much for IE, or browsers that are not modern and don't support CSS. Also its designed for screen resolutions of 10x7. I think as long as you take into account these things, you can make your site however you like
/offtopic
