I can tell you from experience that you have to have a good reason for wanting to do a drugs test. It is an unpleasant and humiliating process. Therefore, it would be logical to deduce that the downside of not being tested is worse than the actuality of the test, and of course, it is. It's career-over (or at least, it should be).
If you are clean, and yet know that you are going to be accused of being a drugs cheat, you want all the ammunition you've got when it comes to attempting to answer your critics.