None. PADI for example teaches the alternate air source should be in a triangle formed by the chin and the bottom of the ribs, which places the DIR "octo" as you put it - dead center.
As an educator you do have the LDS you work for also making their own rules about how things should be done, and MOST enforce instructors to train students with gear sold by that shop. If the shop pushes an alternate air source like Air2 or SpareAir they may require that you dive with it. Besides this argument I can't think of any other.
The position of the alternate below the chin has another advantage - if it is hung perfectly (as it should be) you can take it with only your head and both arms "tied" behind your back - pretty darn cool if you were stuck in a small passage inside a cave, cavern, or wreck!