I don't really think that they are necessary, at least for me personally.
It is more important to press in front, and while standing. This is necessary for the shoulder as well as the top of the chest. The top of the chest acheives an incredible contraction to assist the weight going up, a unique angle for it to be worked. Obviously, it hits the shoulders hard as well.
Anything more, such as the traps ( if that's the advantage? ), can be worked in isolation anyway.
I do behind the neck presses maybe one set per month, just for the hell of it. But I see no big advantage.