You would need 2 resistors: one to drive the opto's LED and another to limit the current on the external LED.
Moreover, power consumption would be big. Using a normal transistor, you just need to provide it a small base current (in the uA range), and it's gain, typically 100+, will "amplify it" to the LED current (20mA typical). The "amplification factor" on the optos, the CTR (Current Transfer Ratio) is very small; it's given as a percentage, the percentage of current out (opto transistor) from current in (opto's LED), and is usually small - 50% to 600% in the case of the optos you mentioned, that is, 0.5 to 6 (compare that with the 100+ of a stand alone transistor). This means that, to make sure your external LED get 20mA, you would have to provide 40mA from the Arduino, because the CTR can be 50% (0.5 amplification).
And you can also find single transistors much cheaper (like 3 or 4c or less) than "equivalent" optos.
So optos are better for when you need to really isolate 2 systems.