There are many 800 radios in command vehicles, EMS supervisors have 800 also. The 480 actually trails the conversation on the 800, not sure why but more likely the 800 is being sent over to the 480, and yes not all talk-groups. From what I hear Admin talk-groups, 6A, 6B, EOC.For hospital and other role calls on EOC, which have been in effect way before the 480 was even built. The EOC radio here at where I work is 800.
To add another reason is that if NYC is expecting to loose the 480 (I doubt it will happen BTW) in 2 years why put more radios on it.