It is essential that they are included in the NRL, because if they didnt have their own team, then they wouldn't be competitive in International games, which is where league is heading atm and trying to strengthen Internation Rugby League.
They arent the Auckland Warriors now they were a defective product and are now known as the New Zealand Warriors. They provide trans tasman rivalry and that is a good thing for the game atm. The kiwis love their football, and the NRL thrives in that country.
I know what you are saying that it doesnt make sense to have a foreign country in the Australian Competition, but it is happening in all sports now and different codes. Rugby Union is a prime example the Super 12 Competition consists of teams from Australia, New Zealand and South Africa, and you look at Union on an international basis and it thrives.
The New Zealand warriors are a successful side in that they obtain crowd support, and are able to bring money into the organisation, the same can't be said about the STORM, yet they continue to be in the top 8 every season yet they can only average 5,000 fans per home game.
If league can thrive in an area, then place a team there, League is popular in New Zealand and it is good for the game on a national and international level.