To be fair, it rule one of persuasive writing to put your thesis at the beginning of your argument and follow with your supporting arguments. As it is, you're changing your entire argument in the second to last sentence.
Eh, when I'm writing on a forum, I tend to use train of thought. I don't really feel I'm changing my argument, more of clarifying that I'm not picking on Ubuntu (as all systems have problems), but Ubuntu has a more significant problem of needing to be better than better. Windows and OSX don't need to be good at anything and they would still keep their marketshare.