Exactly what sort of chance do the other 5 or so parties have? What makes you think that the minority parties are that much better?
Put another way, the last Libertarian candidate is a former prosecutor. Do you honestly think he would have followed through with the Libertarian platform after years as a Republican? I don't think so. Four years may seem like an instant, but that's enough to do a great deal of damage (or self-serving) before getting one's ass kicked out of office-- 2001-2003 is a great and recent example of this.
The minority parties are in a position where they're so desperate to reach the 5% national vote threshold, that they have stopped caring about the voting record or ideology of the people they pick. I've met Greens who say that the democracy movement brewing in Egypt could very well result in another Iran-- this is a party whose platform says self-determination is a key tenet of democracy.
So, yes, we have at least five choices-- one that is politically schizophrenic, one that is allied with too many interests that are against the benefit of the whole nation, and several tiny parties that might not have an office in your district and might not be completely honest with you.
If they can get honest, competent candidates with charisma on the ballot, fine, I'll consider them. But as it stands, the primary reason no one votes for them is because they generally don't fit these criteria-- their entire raison d'election is "I'm not {Democrat|Republican}, those idiots suck".