When the SO thing was started - 15 or so years ago - it was a cost saving exercise. All SO's then had to be young and single and it was a huge break for a 23 year old.
Things have changed. SO's in HKG now cost the company the equivalent - or more if they have kids who get school allowances - than a DEFO on a base. Once an SO has done two years and go on the full housing allowance - which alone is about US $ 9000 a month - it gets expensive.
This is one of the main reasons for DEFO. They actually save money on pay/allowances AND they save on training. When you hire a DEFO its is one training course - when you upgrdae and SO to FO you also have to repace the SO which makes two training courses. So its all about saving some cash and saving some training.
In the process career progression and contracts get violated.