You must not know anyone with a significant speech impairment.
Oh, yes, I do know someone with a significant speech impairment very well, and that person happens to be me. As a result, just about the only time I use Siri is in the car for mapping purposes (e.g., "How to get home?", "How to get to Greg's?", etc.). My wife, on the other hand, uses Siri all day long to dictate long messages (and claims that it saves her one hour of her time each day). So what? I'll use canned responses and if that's not good enough, take out my iPhone to finish them.
What about Siri mistakes when translating? Imagine you start dictating to the Watch (for a reminder, for example) and then when you finish you notice that Siri got a word wrong. What are you supposed to do then? Take out your iPhone and redo everything you just said to correct one word?
You won't have to redo everything. You can start on the Watch and continue on the iPhone (just like Continuity but implemented better). Anything you do on the Watch (messages, calls and email) can be transferred to the iPhone where you can pick up right where you left off.
However, you can already correct Siri on the iPhone and Apple will probably have a couple tricks up their sleeve for doing this on the Watch.