A Maori mate of mine tells me that the haka is a personal and group/tribe thing.
In the real olden days, it was performed as a display of manliness and readiness for war - a real challenge to try and scare the bad guys off.
The bad guys responded to the challenge with their own tribal haka. Again posturing to show that they weren't scared, and in fact it was the home team who should be worried.
A fairly standard bit of a male posturing dance routine performed throughout the animal kingdom, and often seen in pubs late at night anywhere around the world when males have a bit too much fighting juice on board.
In some respects, the original intent of the haka, challenge/counter challenge, has been overtaken by the modern society, particularly when the challengee is not properly attune to the maori/polynesian way of doing things. It has become firstly a motivational tool for the AB's (and any other team performing it), and secondly a marketing dream for promoters of the game.
In relation to the first aspect, it doesn't matter if it is done publicly or in the confines of the dressing room. It motivates and focuses NZ teams. Various teams have pre game rituals. Tommy Rudonikis used to bash up lockers in the changing rooms as part of his pre game ritual. Many NZ Teams have their own haka. Each to his own, and whatever turns you on. In the real olden days, there used to be a macho type contest to see who could give the loudest and "ballsiest" 3 cheers calls. This in some minds was an important factor.
Does it give the Darkness an unfair advantage? No. It is simply a pre game ritual (albeit very important culturally to them). At Test level we are dealing with grown up, hardened professionals, who are not intimidated by things such as this.
Does it give something to the paying public? Ken oath, many demand it, and the Darkness are only too happy to oblige.
The old farts in the IRB should pull their heads in and get on with developing the game worldwide, and allow hosting nations to actually make a profit from hosting the tournament, instead of gouging every last cent that they can get from the RWC host nation to pay for their own personal 6 star presidential treatment and gallons of gin'n'tonnix and pimms.