Third parties in a non-parliamentary government always serve to hurt their own interests. No matter what the messaging or issues or preferences du jour are, they will always end up being more closely aligned with one of the two major parties than they are with the other. And thus they will "steal votes" from that party. The more successful the third party is, the more successful the less closely aligned major party will be.

I'm not familiar with the greens, but have looked into the libertarians. Present Libertarian Party is an unprincipled joke; for a start, the candidate in 2016 did not support freedom of association. Right now, the party looks like a shelter for ex-Democrats that aren't progressive enough for the current climate.

Libertarianism that Ron Paul was pushing in 2008 and 2012 Republican primaries was worth considering (and I personally would love if the guy won), but notice how he had to choose one of the two major parties to get noticed, just like Trump did in the last cycle and other non-mainstream candidates did before them.

A 3rd party isn't going to win any seats. The Libertarian and Green parties are convenient parking spaces for quasi-protest votes. The Tea Party was always the Republican Party. I'm a progressive liberal and I'm a registered Democrat.

We have First Past The Post voting and Duverger's Law says as a result we will have two real parties.

https://en.wikipedia.org/wiki/Duverger%27s_law

Duverger's Law describes tendencies, not inevitabilities. Canada has FPTP and several parties in Parliament. UK the same. A regional split in the party system, or a conservative insurgency against the Republicans in Alabama, or a liberal one against the Dems in DC, or any one of a number of other scenarios would do it.

Eh. Multiparty systems generally form a couple of coalitions anyway. It's not really clear to me what the big difference would be if Rand Paul had an L next to his name instead of an R.

I think the idea is that with multiple parties the coalitions would be more fluid. There's already a soft version of that with "blue dog" and "gang of 8." You do need some sort of coalition to get anything passed.

Perhaps he'd vote against instead of abstaining, since he wouldn't be beholden to the big R?

Breaking the dialectic stranglehold the two party system has on this country would be the benefit. Nothing's perfect, but multiparty gives the system more diverse voices, more opportunity for people to choose and think for themselves.

voting with the minority is the same as abstaining.