Well with USB standards giving each port a max of 500mA. For each device plugged into that port, you're dropping the available power.
That's how power is supplied. Now if I plug a device into hub 2 that pulls 200mA, it leaves 300mA for the other 2 ports, because port 1 can not (not supposed to) exceed 500mA. So if you plug into 3 that requires 400mA... something is going to be hurting.
 - 500ma (1)
L___ - 500ma (2)
L___ - 500ma (3)
My module supplies(makes available) a total of 1500mA. Making it perfect for power hungry devices, even more than one . The other thing is my modules can be set to be always powered on, for items like cell phones, or gps pucks that you would like to keep on...