Leadership comes in a couple of forms as far as I understand, one is leaders that are inspirational / role models, these kinds of leaders in my view are not forming a hierarchy they are providing a service of sorts, a kind of division of labour I guess.
The difference is in the form of resource rights/control/access.
In my view the question you have to ask is does one person have rights over another, if the answer is yes you have a hierarchy if the answer is no you have division of labour, the latter in my view is acceptable, the former is not.
With regard to the "nonviolence" principle, it is a principle that is so ambigious and subjective it is of little use (there are many forms of violence passive and aggressive both are massively subject to interpretation). Also it is unenforceable (without threat or use of some form of violence).
In my view there needs to be the value system in place to reduce any motive that might encourage any form of 'violence' against another to a minimum.
EDIT: As for "is it necessary", ants are an example of no leadership, and it massively benefits them, if you research swarm theory you can see how far greater intelligence can emerge out of extremely limited intelligence, humans could learn a lot from this.
Fact is all systems are emergent, hierarchy emerged because it was the most efficient and effective within its paradigm, once another system becomes more efficient or effective it will become dominant.
Hierarchy and leadership has a number of massive issues and inefficiencies, corruption being one, inefficient hierarchical systems will eventually be replaced by non-hierarchical systems, you are already starting to see this with 'flatter' organisational structures especially in highly technical fields, the more intelligent the work the more it benefits from not having leadership or centralized control.
So basically individual or group leadership (in the hierarchical sense) will eventually disappear, it is inevitable, it is just a matter of time, those in such positions of power and influence are however going to fight it every step of the way it is just a matter of whether we destroy ourselves before that happens.