Depends who you talk to. I will never us a braid designed as mainline as a hooklink material. However, the reality of my fishing is that, where I do use braid as a mainline, it's usually in conjunction with a mono or fluoro hooklink.
Whether you need to use a leader depends on the method, distance, weight being cast and diameter of the mainline braid being used. There's no real need for a leader unless you are trying to chuck relatively large leads a long way on relatively low diameter braid. Some folks dispute this, but I suspect that it's as much down to popular myth as actual experience.