What is the problem with ranked voting? If I were to really want Jill Stein to win, but if she can't win, I'd put my full vote for Hillary Clinton, because I do

What is the problem with ranked voting? If I were to really want Jill Stein to win, but if she can't win, I'd put my full vote for Hillary Clinton, because I don't think Trump should win- I as a voter should have a way to indicate that in a democracy. Simple 1-candidate voting enforces the two party system and makes people pick the lesser of two evils. Without proper democratic voting we might as well choose candidates at random, as at least that would provide a chance of the majority electing a candidate they preferred.