I think the point is that it's harder to see what line number you want to jump to if you don't have line numbering on. . . .
Right, the alternative is to just sort of guess how much you need to ctrl-u/d or j and k to get to a specific line. Also, I guess it depends on screen size. I usually have plenty so the line numbers' usefulness outweighs the extra space required, at least for me.
Yes, that's exactly my point
Yes, that's how we all jump to a particular line :) It's just easier to target a specific line when numbering is turned on, as hesitz said.