Benotti69 said:
Edit. Re-read the OP and edited post.
The English footballer Rio Ferdinand got banned for 8 months for missing a dope test after training.
The whereabouts system allows for 3 violations. Hence why Alex Rasmussen (and isn't there a British runner who was pinged on this too?) were caught.
The reason for it is that it can be quite difficult to guarantee where you'll be at set times, because various things can mean you aren't able to do it. As Voeckler pointed out, usually it amounts to nothing because you're not likely to be tested on any random day. Other times, you miss a test. Other times, such as happened to Óscar Pereiro, the testers turn up at the wrong time, so despite you doing nothing wrong you get strange circumstances.
The difference with Rio Ferdinand was that it was a test that was known about, and he deliberately avoided it. That's why he got the ban.