The only people I've seen (and Ive tracked metrics in stuff like this among large user) using dictation beyond texts and short replies are specialists like attorneys and doctors and people in a bind with injuries.
Personally, i type faster and more accurately than I talk. The 1960 version of me would probably prefer dictation!
We had a long argument about usefulness, because as a Writer, I carry a small notebook and pen. Our joking back and forth went over stuff like "Battery Life" (Phone has a concern, notebook does not) and "Water Resistance" (neither scores that great), "Cost and Maintenance" and so on and so forth.
For my needs - writing notes - the little old school approach won, but it was inferior for his needs.
Disclaimer: I'm working on a knowledge management app thats core focus is connections and tagging (amongst many other useful features)