SMD LED for microprocessor output without resistor
LEDs have a very steep voltage verses current curve as see in the following plot taken from this page:



Notice how fast the current rises once you forward bias the diode (as seen in the green colored quadrant of the plot).  This is why designs include a current limiting resistor in series with the diode.  It is true, if you monitor the voltage very carefully you can likely get by with out a current limiting resistor.  But using a resistor is the easy &amp; safe way to design the circuit.

Also, different color LEDs operate at slightly different voltages.  It would be much easier to design a circuit using different current limiting resistors instead of a circuit using different power supplies, one for each differently colored LED.