Including Gary Johnson means that instead of 1 republican in the debates, there'd be 2. 

A libertarian is a republican who likes to smoke a little weed. The two big issues that differentiate a libertarian from a republican are drug legalization and keeping abortion legal. But libertarians never invests any significant political will or effort in either one. Both these issues are supported by Democrats and when has a libertarian ever at the national level allied themselves with Democrats? The next time a libertarian works with Democrats to stop the mind-numbingly ineffective drug war effort will be the first time.

For more information on "libertarians" who are really republicans, see Paul, Rand; Paul, Ron; Johnson, Gary republican governor.

A libertarian almost always votes republican when there isn't a libertarian on the ballot...which is most of the time. Libertarian is just another name for Republican.