It confuses the results. A "temperature stabilization diode" works in most cases by removing a proportional amount of leakage current from the base of the transistor. Since the leakage of the transistor you're measuring is one of the things you want to know to make a decision on whether the transistor is good or not, the stabilization diode helps cover up one of the things you're measuring.
Every single NASA manned mission starting with the Gemini series has carried a roll of duck tape. Yes, really. Look it up.
Thanks for replies. I just tested it by putting a leaky trannys E-B junction reverse to the socket where other tranny was under test and it immediately gave me something like 17mV, so that lead me to the conclusion that it will throw things off.