The doctrine just a few years ago was that no one single part of the Army fought alone. They were all to use the combined arms doctrine. That is the belnding of multiple weapons systems in concert to overwhelm and destroy the enemy. It also means using more than one service branch to get the job done IE navy or Air Force assets as combat multipliers.
Experiance has shown that sending one group, unsupported, is stupid. The experiance listed in earlier posts is confirmation of that simple concept. Infantry is not the do all, be all, any more than Armor, Aviation, Artillery etc. is when used in a conflict. As much as any troopie would like to think, any one part of the military can't do it all. It is really a team effort that gets the best results. Armor Infantry and Aviation working in concert to control the battlefield.
Helicopters are not tanks, tanks are not impervious and troops can't just get accross the battle field and do it all either. All the services have something to add to a victory and in some cases are absolutely essential to avoid a defeat.