If i don’t want to give a site credit, such as a scraper etc, then i add the x.. if it’s a good site, or my own site, or a site i am referncing, then I keep the x out 
Personally, I don’t care if they follow my link and see that I had something to say about their idiocy, its more that a link is supposed to be viewed as a vote right.. well I’m damn sure not voting for those that I put an x in front of, and I’m not providing them any backlinks either
I think what happens is.. as ppl start to join the forum, they don’t know why the x is there, they see it though, and in being considerate, they follow the trend .. I’ve even seen ppl use the xx infront of their own URL 
~LadyH