If there's an injury during the World Cup, surely the selectors will be ringing Nannes before Pattinson or Starc. Warner has to be 4th/5th choice opener behind Watson, Haddin, Marsh, Paine and maybe even Hopes
Hence they are trying out Pattinson and Starc. If they go well in India then the selectors have more options, if they don't go so well then they know what they will get from Nannes if needed. But really none of the 3 should be needed with Johnson, Bollinger, Harris, Lee, Hilfenhaus, Siddle, Tait, McKay all to call upon.
As for Warner, hes there to give us that x-factor, lets be honest our batting is pretty weak atm, our totals have been sub par and our starts likewise. We are missing that opener to get us away to that flyer, Watson can do it here and there but not consistently. Paine lacks the acceleration for an opener. Hence the selectors are looking at two possible explosive options in Marsh and Warner. Marsh himself is like Watson in being inconsistent with the explosive starts in this format. Of course Marsh might end up at 3 with Warner and Paine as the openers.