I agree that Argentina needs to join either the 3 nations or the 6 nations to develop. The IRB needs to pick a league and force the issue.
Tony Ward (former Irish international) wrote an excellent article on the subject in the Irish independent a few weeks ago. He outlined what the issues were and the consequences of the IRB not doing anything.http://www.independent.ie/sport/rugby/tony-ward-time-to-introduce-promotion-and-relegation-to-six-nations-1166293.html
His conclusion was both competitions need to introduce promotion and relegation aspects.
3 nations becomes 5 nations (Argentina, S. Africa, New Zealand, Australia and the pacific island team with the best record over the previous 3 seasons).
6 nations becomes 7 nations (Ireland, England, Scotland, Wales, France, Italy and the winner of the European Nations Div 1 cup the previous year). European Nations cup includes 35 countries with the exception of the 7 nations.
After a decade or so, if one team dominates from either Pacific Nations Cup or European Nations cup, they like Italy/Argentina could become permanent members of the major tournaments