Andy Fitzgerald's presentation, "Taxonomy for App Makers", doubles as a solid exploration into taxonomy as a concept. He presented the material at the 2013 I.A. Summit in Baltimore, and he's synched the audio from the talk with the slides.
Information architecture helps avoid waste and capture maximum value earlier in the development and test process. Agile and lean teams should always begin projects with information architecture for any project that will run eight weeks or longer.
Ultimately, not every project needs information architecture. Or design. How well you understand the problem and the solution, as well as how that understanding changes, determines whether or not you need architecture, design, development, or testing.