The premise behind open borders is that free people should be allowed to travel unencumbered. If it were not for our vast welfare state put into place by republicans and democrats, immigrants would have no reason to be here unless they wanted to legally work. We already have open borders. At least under a liberarian government, the people coming here wouldn't be freeloading at the expense of taxpayers.
There was a time not all that long ago (early 90's) when I thought perhaps the Libertarians had potential, but after doing some serious research I discovered what they really are as they pretend to wish to return the country to what it was founded on. The Libertarians IMO are nothing but an amoral bunch of lefties who don't want to admit to being Democrats.