Since the trundle bed is very space efficient, it is also flexible with the size of the bed. You could have a large main bed and a smaller, single pull-out bed. Both beds could also be the same size. This space-saving feature is very suitable for small Hong Kong apartments.
Additionally, if you ever need an extra bed for a relative staying the night, the need is temporary. The trundle bed is easily stowed away, so it's only taking up extra space when in use. If you want to have more kids in the future, the trundle bed may be a good starting point before they move to a bunk bed or other style bed.