Human-human spoken dialogues are considered an important tool for effective speech interface design and are often used for stochastic model training in speech based applications. However, the less restricted nature of human-human interaction compared to human-system interaction may undermine the usefulness of such corpora for creating effective and usable interfaces. In this respect, this work examines the differences between corpora collected from human-human interaction and corpora collected from actual system use, in order to formally assess the appropriateness of the former for both the design and implementation of spoken dialogue systems. Comparison results show that there are significant differences with respect to vocabulary, sentence structure and speech recognition success rate among others. Nevertheless, compared to other available tools and techniques, human-human dialogues may still be used as a temporary at least solution for building more effective working systems. Accordingly, ways to better utilize such resources are presented.