Well,
it WAS totally okay for about 6 to 8 years. There was no Supplement to the AFM for the B200 (that I flew then) and the Authorities just wanted a Procedure to OM-A and D for the use of London City. I wrote one, containing all necessary information for the training and use of LCY - granted no information wether the KA could actually do the trick was available.
(We trained on the actual aircraft with VNAV capable FMS and indication via the EFIS PFD)
Everybody was happy: in our case LCY Airport, the CAA and the LBA (german Authority).
Landed in LCY maybe 25 times, give or take a few times...
Then somebody discovered that there are Supplements to the AFM for steep approaches.
IMO the situation is: well we know for sure the KA can go into LCY, likely thousands of movements have been done without incident, but now we need to prove what we already know.
IF there would be any doubt to the capability to perform that type of approach on the aircraft I fly, I´d certainly won´t do it without the prove...
If you ever flew a B200 (or any other PT6ed KingAir) and you pulled back the throttles and had the props full forward you know that she could go even steeper than 6 degrees. Its like an elevator going down.