The idea of a reference track is to make sure that you keep your ears tuned to what makes a good mix.  When you’re mixing, it’s easy to lose your perspective – by referencing other material, you keep your ears fresh and stop them from becoming attuned to a mix that’s getting more wayward with each EQ tweak.  To make sure you get good results, though, your references need to be free from flaws themselves, but if you’re using MP3s as your reference source, that might not be that case.  To illustrate what I mean, in this video I use Voxengo Curve EQ to compare the frequency characteristics of an MP3 and WAV of the same song, to see what difference the MP3 encoding has made to the original recording on CD.