Do I need a motor shield?

Sorry to ask such a beginner's question, but I can't figure if I need to purchase a motor shield or not to drive a simple robot or can I get by using 1 or 2 H-bridges?

Currently, I'm able to drive motors with my Arduino Uno using an H-bridge as described here: http://itp.nyu.edu/physcomp/Labs/DCMotorControl. In this particular case, there is one H-bridge which can drive up to 2 motors but I assume I can also use a second H-bridge to connect two more motors (4 in all for a 4WD robot)? Also it looks like I can connect an external power supply with a H-bridge. So then what is the main advantage(s) of buying and using a motor shield which seems to cost a lot more than 2 H-bridges? Thanks!

awawda: So then what is the main advantage(s) of buying and using a motor shield which seems to cost a lot more than 2 H-bridges? Thanks!

The motor shield plugs right in on top of the Arduino -- it's just a convenience.

Strictly speaking the L298 on the motor shield is capable of a bit more amperage and has current sense outputs (tells you how much power your motors are using).