Metonymy is the least interesting way our minds work.
Metaphor is a much more interesting cognitive pattern. To use metaphor, we must notice a way in which two things are similar. This is a difficult task. It's true, an African grey parrot can do it, but when we see him do it, we are rightly amazed. Metaphor requires an understanding of abstract relationships, separate from the things themselves. "I dissolve connivers like saliva on tic tacs." "Shake it like a polaroid picture." (And of course the conceit underlying Rich Boy's section of the Diplo Street Remix of M.I.A.'s "Paper Planes.")
Compare this to metonymy, which is the linguistic phenomenon of calling something by the name of something associated with it - and represents the cognitive pattern of noticing that two things occur together. It's pretty primitive. (Think Pavlov.) E.g. "Two to the ski mask." "All I love's my dope and dead presidents." (Synechdoche is a species of metonymy, the part associated with the whole - e.g., "Get your ass on the floor." Or "pussy" or "gash" as collective nouns - though those are metaphors when used to denote an actual vagina.)
Metonymy notes correlation - proximity in space or time. Metaphor requires us to form a theory about how things are similar.
You can see how both would be extremely useful, in terms of evolution. Metonymy is easier, and it goes lower down the phylogenetic ladder (<--metaphor). Both processes can, of course, get things wrong. Metonymy, however, is such a dangerous temptation for lazy human thinking that we have had to invent statistical analysis to get science to work.
Unfortunately for our species, metonymy seems to dominate political thinking. Bad things are allowed to "contaminate" metonymically anything associated with them - even if not similar or even rationally related in any way but association. Why do most people oppose eugenics? 'Cause the Nazis did it. Similarly, good things are allowed to "rub off" on associated phenomena, e.g., "the family" with anti-gayness since same-sex couples can't procreate (well, by themselves).
Metonymy, I think, is a prime driver of bullshit – a pretense to truth minus any actual concern for truth. Metonymy is the way in which mere facts can seem dangerous. When we are candid, we admit as much. It is uncomfortable to entertain the hypothesis that, for example, the etiology of homosexuality might be in some way environmental or volitional, because that is associated with the claim that homosexuality is wrong, and hence with retarded attempts to forbid it, “cure” it, or otherwise persecute gay people. It is uncomfortable to entertain the hypothesis that the mental abilities of men and women are different, because that is associated with the practice of female subjugation.
If we are to think well, the challenge is not to get rid of metonymy, but to root out knee-jerk, unexamined metonymy. “The Nazis did it” is not an argument against a practice. Nor is it a legitimate challenge to a factual assertion to point out that the fact might be used to support a nasty conclusion.