@ It is a symbol of excellence, reflecting the overall professionalism of the entire Army.
@ It was intended to unite the Army, which Shinseki felt was deeply fractured by the existing berets (red for airborne, green for special forces, black for rangers). Ironically, the only unity it has created is the near-universal hatred of berets.
@ Since black berets are (everywhere but the US) worn by armor troops, and in the US by Rangers, it was felt that a black beret would reflect a merger of both light and heavy forces.
@ Selecting the color black was not intended to insult the Rangers (who are deeply insulted and will always believe that Shinseki's move was intended to reduce their stature) but was simply the best color to match the battle dress uniform.
@ Above all else, Shinseki wanted to create a "world class uniform" that would be respected by foreigners who came into contact with US troops.--Stephen V Cole
General Shinseki recently authorized his deputy Chief of Staff, General John Keane, to explain his decision to put most of the Army into Black Berets: