If a PIC MCU provides multiple Vdd/Vss should you provide power to them all?

Hello, all. Please take a look at this example schematic: 
 

The chip in question is the PIC18F4550 and as you can see power is provided on both sides (with a 100nF cap to smooth noise I guess). Is this strictly required or could you put power into one side and just ignore the other? I know that I have just put power in one side and everything still seems to work fine but I feel uneasy with doing that. I feel that they would not waste pins that could be doing something useful by having an extra pair of power in/out pins. 

So my question, at its heart is: for what purpose does this PIC MCU (and others like it) have multiple power pins? Thanks in advance.

Power and bypass (with the 100nf caps) both sides. You want one of the caps as close as possible to the power/ground pins on each side.

The chip may work with power only being applied to one set of inputs, but running outputs at high currents (like driving leds) will likely strain the internal power distribution to a point where you will get failures. Powering one side only can also have significant impact on clock oscillator stability and accuracy as well as ADC analog performance.