Same as Jill Stein and the Green party. Both Stein and Johnson are as far as I am concerned far more interesting candidates than Romney or Obama. They aren't corporate financed though and don't own any news organizations and our system is set up to make third parties a near impossibility. Not letting them in the debates, for example.
I wouldn't necessarily say Johnson won't be a factor. While he's polling low overall, I have seen some polls which have him between 7-13% in the Mountain West. It's hard to say whether those votes are taken from Obama or Romney or if they are simply people who wouldn't have voted for either anyway, but it potentially is enough votes to alter the outcome. As an example, McCain won Arizona by 9 points, which is 200k votes. That's not a lot. it's possible Johnson could pull those votes from Romney and turn Arizona from a solid red to a toss-up. I wouldn't discount him from having an effect in other tight races as well (e.g. Florida). Some could be so close that a couple percentages can make a difference
Put Johnson on a stage with Romney, and I'd bet a whole gaggle of Republicans would say, "holy smokes, that's the guy I've been dreaming of, let's dump the Grey Poupon guy." I think the same sort of thing would be said of Stein as well. It's easy to say they aren't a factor when we don't give them a chance to actually be a factor. Put them all up on stage and let people decide for themselves rather than choosing between the two guys who have the most money to spend.