But if a political party is never given the opportunity to have their platform widely heard then how will people who might actually agree with the platform know this and know to vote for them?
It's not like the average citizen gets to participate very meaningfully in democracy. They get to vote once every 3 or 4 years for one of two indistinguishable platforms. If they oppose a war, or oppose healthcare cuts, or oppose the feds giving tax payer money to religious schools, well that's just too bad, cuz both parties agree on these issues.
It might be messy, it might be unwieldy, but for fucks sake, why would you ever oppose anything that gives a greater voice to the vast majority of the citizens of your country, who are otherwise essentially mute.

That just seems plain undemocratic to me.