You cannot be selective about evidence. Frigo, for example, was hurt by what happened to him and threw a lot of accusations around that have not been verified. Pantani admitted being a user (as did Riis) and so that is a sufficient burden of proof. Other riders mentioned in this thread were of a previous era and people like Walsh made some very powerful arguments about custom and practice in the peloton at that time. However, there is now a huge movement towards a clean sport. Agreed, the sport (no sport) is not 100% clean but, my God, we have come a long way forward since the dark days. I dare you to stand up in front of Cavendish, Stannard and Wiggins (to name just three) and accuse them of doping. I think you'd be flattened very quickly (either physically or verbally!!!). Confront David Brailsford about systematic doping at Sky and you'll be in a court of law before you could say 'epo'. There is no evidence that systematic doping exists in the sport as of now. Of course, there are unscrupulous individuals who will always try and buck the system in any walk of life. Were it ever so?
There are also false negatives because the science of dope testing may, just may, still not be up to scratch allowing errors to occur. That just makes the waters slightly murkier. Zirbel is a case in point. I wouldn't say that I 100% believe that he doped.
And where do you draw the line at doping? I have just edited an article about someone's personal best time trial where he talks about 'necking a caffeine gel 10 minutes before the off'. Tell me guys, is that doping or not? I was very uncomfortable reading that, so believe me I'm not an apologist for cheats. No way.