The port expander only uses a small amount of power when in use, but if you are powering it using the 3.3 V supply, then you will still only be able to draw a maximum of 50 mA in total from all the pins. If you draw too much power, then you may experience system freezes or corrupted read/writes on the SD card.
If you power the expander using the 5V supply, then you can draw up to the maximum power the expander can support (around a maximum of 25 mA per pin and 125 mA in total), as long as your USB power supply is powerful enough.
We must remember that if the expander is powered with a 5 V supply, the inputs/outputs and interrupt lines will also be 5 V and should never be connected back to the Raspberry Pi ...
