Can't see what all the fuss about timings is about.... departure not much later than CX out of HKG. Just check the schedules for a few Asia airports and you will find numerous departures between 23.59 and 02.00. From experience, these often have LF's around 90% or more so timing does not deter Asian passengers.
From a personal point of view, this departure time lets you have a full day of meetings in Beijing (or elsewhere in China if connecting through PEK), a leisurely dinner and a shower in the hotel, and then a taxi to the airport when Beijing's appalling traffic is at its lightest.
Some people are never satisfied... the world is never going to be perfect, and Hainan need to fit this service into their schedules to ensure high aircraft utilisation. It's better to have this service than none whatsoever (and I am certain that Birmingham would have welcomed this service at whatever times HN was prepared to arrive/depart)
Anyway, I would hazard a guess that the vast majority of those making comments about the timing of this flight will be people who will never use it anyway!