Most serious macro folks these days do photomacrography rather than photomicrography, even at traditional micro magnifications, using microscope objectives. The primary reason to use a microscope is if you need bottom illumination (brightfield or bright/dark field), or sophisticated lighting techniques (DIC/Nomarski, fluorescence, etc) to get the best contrast from transparent and/or living tissues. Coins don't need any of these special techniques, and in fact the highly flexible brightfield illumination needed for coins is difficult to achieve within the confines of traditional microscopes.