"There are life principles, there are work principles and there are electrical design principles." 

I have seen numerous electrical setups. I have seen some electrical system setups driven by lack of knowledge, I have seen some driven by ignorance, some driven by lack of money and some driven out of carelessness. No matter how you look at it, most electrical technicians have a reason they chose to deviate from core electrical standard principles. 

A multi-floor building, should always have more than one distribution board and thus it should have a Sub-Main Distribution Board (SMDB). Rather, what you see are some anomalies. First, the number of floors in a building is never equivalent to the number Distribution boards in that building. This means that a 3 storey building which actually means a G + 3 floors building should have 4 numbers of Distribution boards. However, what you see is the Client or Electrical technician or Electrical installation company cutting cost by going for 2 numbers of Distribution boards. Going for 2 numbers of Distribution boards can still work. However, it may lead to possible hiccups and installation difficulties. Below are the core reasons I advise that DB's be installed on every floor of  a building:

1. Limitations of the Cable tray
Most times your Cable trays are design to carry only load cables. Cable trays are designed to carry load cables from the Main panel of SMDB to the Distribution boards. They should not be designed to carry distribution cables. Distribution cables can be many and in no time will fill up any available space in the cable tray.

2. Difficulty in troubleshooting for faults

3. Voltage drop becomes a reality