While I am not an advocate of open borders myself, as a libertarian I can certainly understand the argument. What I cannot understand is under what theory people who have crossed whatever border into the United States would become entitled to anything at all, whether money, food, clothing, shelter, education, or medical care — to say nothing of the “right to vote.” There are of course purely humanitarian considerations, and I see nothing wrong with saving people who would die no matter who they are or what their status.
I just wish people would not be assuming that “open borders” is an argument for granting citizenship to anyone who manages to come here. They are two entirely different issues.