Now that you mention it... what happens if the Discourse user changes their password on Discourse, then uses the "Log in with Discourse"? Does it fail on a password mismatch, or does it update the WP password?
Another suggestion: Could you make the "Log in with Discourse" text configurable? Our users aren't necessarily aware that the forum runs on Discourse, so this may cause some confusion. If I could configure the text to read "Log in via Jag-lovers" there would be (marginally) less confusion.
The account creation email is just a nice to have.