"Rather they will increase public participation and inject new, imaginative ideas, into a system that has grown unresponsive, hidebound and sclerotic."
No, they won't. There's no logical basis for thinking that a third party will behave in a fundamentally different manner in that respect than either of the two current ones. I suspect it comes from having an unrestrained "imagingation".
"Really, what good is the system we have now if it is not doing its job."
Our system's job is to provide political order while not oppressing the population. By that metric, it's doing, by historical standards, a fantastic job.
"Parties which truly represent large segments of the American people and are not part of the current monopoly are far more likely to listen to their constitutents and less likely (not NEVER, but less likely) to be corrupted by lobbies and special interests."
A party that represents a large segment of the population, but not the majority, is a special interest, If you think that more third parties would be good, but that special interests are bad, then you haven't thought about this nearly as deeply as you need to before trying to argue on it's behalf.
"special interest"
There is no ATLA political party, no NEA political party, no Pharma political party, no insurance polticial party, no NRA political party, no AARP political party, no US chamber of commerce political party, no tobacco political party, no NARAL poltical party etc. etc. These alphabet organizations are not parties, they exist to funnel campaign contributions (i.e. "buy") "demopublican" people whom they like, not to get THEIR OWN MEMBERS to office.