Why is it so difficult to use a "true mirror" as a mirror
This answer supports the comment by Krysta that we are simply used to the mirrors we have and could just as easily learn to use a "true mirror".

In 1950, Theodor Erismann and Ivo Kohler performed a famous self-experiment in Innsbruck, Austria. Kohler wore a pair of glasses that turned his view of the world upside down continually for 124 days (sic). After about 8 days his perception had adapted and he was able to ride a bike or paint just like before.

The image below shows Kohler wearing the glasses:



Here is the original documentary movie created by the two researchers (in German) on the website of the Center for the History of Psychology at the University of Würzburg, Germany: http://www.awz.uni-wuerzburg.de/archiv/film_foto_tonarchiv/filmdokumente/th_erismann_ikohler/die_umkehrbrille_und_das_aufrechte_sehen/

Here is an article from the German Wikipedia describing the glasses and showing an image of a current (self-built) model: http://de.wikipedia.org/wiki/Umkehrbrille I don't know what this "Umkehrbrille" ("turn around glasses") are called in English.