People vote for the Right-To-Life candidate, for example, because that issue is very, very important to them and their conscience compels them to cast their vote that way. This is a poor analogy. Pro-lifers see abortion as murder and is a moral absolute to them. On the other hand, many Libertarians feel there is some theoretical ideal they're voting for which in its totality has NO chance of ever being adopted, and which a normal person would not consider a moral issue on the order of abortion.
For instance, many people includng myself would like to see the Dept. of Education eliminated. However, if a Republican candidate who can actually win is not for eliminating it, but is still clearly differentiated from the Democrat on a number of issues, and a Libertarian who has no chance of winning wants to eliminate it, how important of a consideration is their DOE position really? It would take a really peculiar person to vote for the Libertarian based on his abolish the DOE position regardless of what difference do exist between the Dem and Republican, yet that's what Libertarians do.
If a Right-to-Life voter tells me she's voting for that party's candidate because the issues the candidate stresses are very important to her (and it doesn't have to be because she believes abortion is murder, it could be because she thinks the issue belongs out of federal hands) and a Libertarian voter tells me the same thing about the issues her party's candidate streeses, I take them at their word and think the both are quite justified in voting as they do.
Might doesn't make right and just because the two major parties typically get more votes than other parties doesn't mean people must be reduced to having to vote for one of them (the Soviets had one party, we can do better than having just one more than they had). Major party voters sometimes get upset that their candidate didn't win and wonder why others voting for candidates in other parties didn't vote for their candidate. But those other party voters can wonder the same thing about major party voters.