I agree with you, it is meant to be rapid prototyping, but it is not necessarily so. It really depends on the part you're making. If you consider the time it takes to design the item in 3D, adapting the design to the specifics of printing in 3D and your actual 3D printer, and the time to print the thing, it can take an awful lot of time. Sometimes, people would print in 3D something that can easily be done using classic machining -- or with clay, cardboard or papier-mâché, just because they can, it's fun or it's hype not because they actually need a plastic part or is faster.
What 3D printing is really good at besides highly complex shapes is rapid-iteration prototyping, where you print several iterations of a prototype with minor changes between each iteration. Then, you start saving a lot of time.