This is a good point. I can't comment on Plan9 or Xiki, but I believe this was how Linus went about developing Git. I remember watching a video where he discussed his complete ignorance about CVS and SVN, and how that was a blessing.
Having said that, it is a bit disheartening when somebody solves a problem which is solved almost exactly the same way as several other solutions.
Um. And he was not so ignorant of BitKeeper. I also had the impression that it was not so much about ignorance of CVS and things as it was about disliking the way they did things. But I would not know for sure...
Not that I would tell these guys to go about their business this or that way or anything. Whatever works for them and so on. Though it would obviously be helpful to know which things it is similar to and how it compares to them when telling people what it's like.
Having said that, it is a bit disheartening when somebody solves a problem which is solved almost exactly the same way as several other solutions.