Sure, but more and more Siri and Google Now are doing that training. And having used Dragon, I can say it's a very heavyweight solution that requires a great deal of training, not just of the system but of the user. I would vastly prefer a lightweight mic box next to each input field.
I think on mobile it's lost to the keyboard as well as a general purpose input tool - the hardware and networks weren't there to support it initially, on-screen keyboards have (massively) improved and people have grown more familiar with them, and devices have grown considerably as well which allows for an even better typing experience.
