In this case, quaternions have been shown to effectively solve problems of 3D rotation and interpolation. Perhaps they have, and we just haven't heard from them. Quaternions lend themselves well to graduate projects and technical theses. Vectors, by comparison, are straightforward to the point of being boring. There is not much new to say about vector operations in R 3 .
Quaternions are a popular buzzword.
And their intrinsic difficulty is appealing. Only the most focused, deep-thinking programmers are able to understand and use them. But that is not enough to ensure that they are the right tool for the job. Sometimes it takes an open mind to look beyond the hype and consider the mathematical alternatives. As Mr. Brown would have admonished, you are not seeing the forest for the trees.