>> I'm not sure a multiparty platform is any better
I live in a country without an entrenched two-party system, so I feel qualified to comment here.
The difference to me is this : if I were a Green in the USA, I would have no voice, because my party is not R or D. As a Green in NZ, I DO have a voice, because even though it is small (~10-12 percent usually), it is still big enough to make a difference in the coalition negotiations. If a bigger party needs the Green's support to rule, as they often do, they must throw the Greens a policy bone now and then, which they do. Smaller parties have a smaller voice, but it's not a silent voice.
The mistake your making here is that if you vote in an election in the US, that your voice really matters. You could vote, r, d, mickey mouse, or count chocula, and in the end, the only voices that matter are the ones writing the checks (the Kock brothers, Wall St, the Silicon Alley sharks, etc.)
This is a product of the two-party system. The two parties serve to give the appearance of debate and choice, but you get no say, in all those things they agree upon. As Chomsky has said, there's really only one party in the USA, the business party.