It's actually a very good question by the thread starter swamped by a bunch of idiots who have nothing to say.
One more scenario, real story: a Russian citizen got a tourist visa, but then they announced 90 days for Russian visa-exempt starting in November. So she's now wondering what to do about the visa as, obviously, going through 60+30 hassle would be ridiculous when you can get 90 days right away.
Update: some non very bright individuals claimed that immigration officers will see your visa in the system which is utter bull - when you have a visa in your passport, it's essential to tell the officer about it, otherwise you might end up with a visa-exempt stamp. Me and my relatives have been stamped in wrong numerous times, even giving them the passport open at the visa page fails quite often.