Compare to window live spaces, I saw most profile pages of myspace&friendster are set these type of background. is this because people like it , or just because the CSS is easer to build(they don't need to combine bgimage with bgcolor.
I'd say because it's the trend for those sorts of pages. Geocities circa 1997 was full of them. Position:fixed has one huge drawback: it doesn't work on anything except the body in IE6 and under. In CSS with any other browser you can set it on anything large enough to scroll (or with forced scrollbars). So because it could only go on the lowest layer, people use it for that and that only. There are hacks/tricks around this problem-- to read about them Google "Complex Spiral Demo" and you'll find Eric Meyer's page where he addresses a way around this problem.
Done properly, it can be a pretty handy design tool - especially if you want to use large complex backgrounds and change them often.