The US Army is in contravention of the Geneva Convention by flying Medivac missions over hostile territory that have not been pre-noted and coordinated with both sides of the conflict. There is a fundamental misunderstanding of what the Convention requires.
The Geneva Convention does not restrict armed helicopters from also collecting casualties; therefore the use of armed and unmarked (ie no red cross) aircraft by the RAF, USAF and USMC is perfectly legal. They use the protection of their weapons to stay safe, rather than rely on a red cross to persuade the enemy not to fire.
The US Army can only fly into a fire-fight with armed escort, thus limiting their response options and introducing a dependency on others. This could add a delay to the operation while an escort was organised. Michael Yon gives proof of this happening and the resultant delay causing the unnecessary death of a patient. His crusade is to stop this happening again.
Speed is a life saver. The faster a casualty reaches help, the greater the chance of survival.
There is anecdotal evidence that the Taliban are aware that helicopters with red crosses are unarmed, and so draw fire. There is also a suggestion that the crosses are seen as a symbol of Christianity and provoke attack.
Unarmed helicopters have greater casualty carrying capacity as they do not have the extra weight of the weapons and door gunner.
One thing is fact – there is no question of the bravery and dedication of the crews performing these acts. Heroes, one and all.