I thought I'd take a break from discussing why Donald Trump shouldn't be president and discuss why Gary Johnson shouldn't be, either. Both men are proposing things that I think would be horrible for this country. The main differences are that (a) Johnson is at least fairly open and consistent and (b) Trump will carry some states and so has a chance of winning the election.
I see out there two reasons why people are saying they'll surely/probably vote for Johnson. One is to "send a message" that they're not happy with either candidate from the major parties. I think it's already pretty well established that most of America would rather have other nominees. That seems like a pretty feeble message to me and a vote for Johnson could easily be interpreted in other ways, like: "Hey, even in a year when most Americans distrust the candidates from the two major parties, only this many [not enough to get a single electoral vote] bought into the Libertarian agenda!"
The other reason, of course, is that you agree with what Gary Johnson stands for and want to endorse all those things. Before you're sure you do, check out what Kevin Drum says Gary Johnson stands for.