There are a few applications for multispeaker models. One is for training a small dataset that is too small to even work on aitch or crust. If you train a small dataset alongside one or more bigger ones, it will be able to learn to speak. Another application is multilanguage training. You can train a model to speak in more than one language by training different language datasets, or you can convert a dataset from its source language to a new language by transcribing it accordingly and training it alongside a complete dataset of the target language.