In most cases yes. In a really close race voting L might result in an R winnning instead of voting D.
Imagine a race with 100 voters and three candidates, R, D and L. Say that 45 voters vote D, and 46 voters vote R. If the remaining 9 libertarians vote L the R wins. If they vote D the D wins.
In other words voting L subtracts one from the R total, but doesn't add one to the D total. In a very close race this might allow the R to sneak by.
Do you really think libertarian's don't vote?
I favor smaller government, want the government out of my personal business and the legalization of marijuana. It's been 32 years since I last smoked a joint and even if pot was legal now I wouldn't smoke it because I don't like the way it makes me feel. Does this make me a libertarian?
But wait...I don't want homosexual marriage, I don't like abortion and I support the WOT.
Guess I don't fit in with the R's though because I don't want morality legislated.