It all depends on the situation and personal preference. I liken using a travel agent to using an accountant to do your taxes. If it is really simple and you enjoy doing it yourself, no need for either. The more complicated it gets or the less you personally enjoy doing it, the greater the reason to get help.

The greatest value of a good travel agent comes if something goes wrong - flight delays, cancellations, etc. They can save your trip and and a lot of money.