It's about more than voting blocks. In a 1975 interview with Reason no less a conservative than Ronald Reagan said, "If you analyze it I believe the very heart and soul of conservatism is libertarianism." Now, obviously the power dynamics and constituencies are different these days, but the truth is the more Republicans win, the more the philosophical backbone of the party seems to weaken under the understandable if not particularly admirable desire to hold onto power. So libertarians may not win Republicans elections, but it's difficult to look at the behavior of this Congress and White House without getting the sense that we could use a little more principled opposition in the ranks. Right-leaning libertarians carry more worth than a simple vote.
Here's more Reagan from that interview, by the by: