NewUser is a clever asset used to drive wedges in the minds of nascent board lurkers so that they think libertarianism is irrational and abandon adopting its ideals before looking further into things.
Eventually each thread is hashed out in detail to explain the sometimes complicated questions posed by NewUser, and the principles of libertarianism become clear after some posts.
However, I positive repped him at least once. I like when RPF diverts away from being an echo chamber where the same opinions and viewpoints get repeated over and over again.
That being said, unless NewUser is intentionally being a consistent devil's advocate by posting threads that use a Socratic-like method to teach people that libertarianism has an answer for X, Y and Z edge cases, then it can be assumed that he is definitely not looking out for the best interest of the libertarian movement, and is actively seeking to undermine it in a subtle but conclusively obvious way. It's kind of entertaining to me personally, but I'm unsure if his presence is a net benefit for it.
This is not necessarily a bad thing. Note that the libertarian movement is not the same as the liberty movement in all cases.