I agree, if your just going from "A" to "B", skip the agency and their fee. However, if the trip is going to take you bouncing across the globe for several weeks a good agency can set up flights, hotels, and car rentals with so much more ease. Regarding multiple destinations, if you have the time and desire, you can DIY, and maybe save some $$$ but sometimes an agency can put together a "package deal". Enjoy!
Brian