The advantage of stepping if you need a large pitch is that one can achieve an optimal growth rate if the starter pitch rate is within certain limits (also discussed in the book "Yeast"). Within that pitch rate the yeast will grow at their highest rate while maintaining a healthy growth environment.
The disadvantage of stepping is vessel size limitations (for some, myself included), time, and risk of infection. If sanitation is at the forefront of your brewing operations, infection should not be a problem. Stepping does require crashing and decanting to remove the spent wort leaving only the healthy slurry, and a significant increase in the volume is usually a good idea, 10 times is recommended but I've had good success with much smaller second steps.
Making a starter is as much about increasing the cell count as it is about creating the healthiest pitch possible. Starting with a large volume such as 4 liters will grow yeast, just not as much as a 2 liter starter, assuming one vial or smack-pack is used.