Jacob T. Levy at Bleeding Heart Libertarians argues that Gary Johnson helps Clinton more than he helps Trump:

And this is the pattern in almost every poll I’ve looked at that compares the 2-way and 4-way races. It’s the pattern in a large majority of states including most swing states. It’s the pattern you would expect from two former Republican governors running a ticket against a deeply unpopular (und un-conservative and unhinged) Republican nominee. It’s the pattern you would expect from the fact that (at this writing) six traditionally-Republican newspaper editorial boards, including the very conservative swing state New Hampshire Union Leader and the rock-ribbed Chicago Tribune and Detroit Free Press, have endorsed Johnson. I’m happy that Johnson and Weld have run a more leftward campaign than Libertarians usually do, and have emphasized Libertarians’ liberalism on drugs, crime, policing, imprisonment, and war. I’m happy that they’re bringing some young liberals into the movement, and I think that’s good for the future of libertarianism. But it’s still the case that the low-tax, free-market, free-trade candidates running against a protectionist Republican consistently bleed off a few more disaffected Republicans than they do Democrats. And the punditry, commentary, and strategy that pits Clinton’s interests directly against Johnson-Weld’s is mistaken.

I find the argument convincing, and I wouldn’t have the chops to challenge it if I didn’t. But…

…If Johnson (and Stein) weren’t running, and Clinton and Trump were the only ones on the ballot. I’d vote for Clinton hands down. My (potential) vote for Johnson will in my case take a vote away from Clinton. That might not matter in Sangamon. I haven’t really looked at the polls, but Sangamon tends to go Democratic and strikes me as anti-Trump anyway. Still in my case, if I vote for Johnson, that will be a loss for Clinton.