I don't believe that my vote for Johnson is a vote "for" Johnson. No way he'll be elected. But, if he gets a major percentage of the votes (15%, 20%, 10%, something), then it will show that a 3rd party candidate is perhaps a viable solution and/or alternative to terrible nominations in the 2 major parties.
For instance, we've heard that the Bushes aren't voting for Trump (rumored). What if Jeb had run as the Libertarian candidate? Would he be leading Trump? Maybe. Not likely, but maybe. And, it may have encouraged more Dems to vote for Jill Stein (because they wouldn't feel that their vote was needed to have Clinton beat Trump, because Trump wouldn't be a shoe-in amid a split Dem party).
Long story short - I want more choices than 2. While I don't think Johnson would be a great President, a vote for him is my way of saying that in 2020 or 2024, I hope there will be 3-4 parties viably running for President.