I'll refrain from making any comment on the piloting or flying side of this issue, but will comment on the building side.
Escape core design is about getting people into a 'safe' area (the escape core) and hence out of the building as quickly as possible, with little thinking being required of the people using it, by means of a fixed stair rather than lifts etc).
Current regs, certainly in Western Europe and the US require escape stairs to allow people to go down and directly exit the building at ground level, with a separate set of stairs leading up from the basement to ground level exit.
This aims to ensure that people 'know' that if they go down the escape stair they will be (in theory) able to escape at ground; not be lead down to the basement. Similarly if they go up from the basement they will exit a ground and not climb to the top of the building.
Now add a potential roof top escape possibility. If the same stair serves both ground level exit and roof top exit in an emergency evacuation you would be trying to deal with opposing flows... which leads to congestion, reduction in flow capacity, confusion etc etc, all of which conspire to reduce the possible safe exit strategy for that building. This would massively negate the additional evac capacity a roof top evac via helicopter could bring (Remember that ground level exit is calculated in 100s and at times 1,000s of people per minute...)
The current logic would be to add another stair case, which is only to be used to access the roof. The challenge is then how do you make sure that people know which is which in an emergency, in a highly fool proof manner? A very difficult issue. (Remember this means it needs to be comprehensible by hysterical people, who are totally unprepared for emergency action, both physically and mentally and who lose all power of 'reasoning' when that fire alarm goes off...for example we have seen people unable to understand that lifts are out use in an emergency, the stair is to be used, yet they stand there hysterically pressing the call button...refusing to use the stair.)
Similarly the flow rate is a massively difficult issue. How many can get off the roof compared to the number arriving? Is there a 'safe place' for them on the roof? etc etc. Imagine 100 people arrive up there per minute, but only 15 can be lifted off per minute.. what to the other 85 do? And the next 100 who arrive in the next minute, with only another 15 being lifted off? Leaving 170 people waiting? And so forth...it becomes a space issue as much as a people management issue.
This is in addition to the space take issue of another staircase within an already area-limited building, which affects the fundamental building viability. This in turn affects whether or not high rise buildings get built and hence the potential need for this type of evacution option.
So whilst there are examples of roof top or balcony rescue saving lives, which naturally provides great support to this additional resource, the step to making it a fundamental part of the overall strategy of building evacuation needs to be considered on both a number of people scale and flow as well as overall operational issues to allow safe access to the roof, without reducing the overall evacuation capacity of the building.
This is not to say that Fire Departments should not have helicopters as part of their resource base, but their role needs to be very carefully considered.