It's interesting that Siri still asks when you want to be reminded.
That was the subject of a lot of complaints on the Apple user forum as Siri stopped doing that in August. https://discussions.apple.com/thread/5251434?start=0&tstart=0
Have you tried it recently?
While Siri works wonderfully for my various lists, texts, email, etc., I am rather miffed that I have to do a work around to get her to add a reminder for a specific date/time.
It depends. If I say "Remind me to call John", she asks which one, then when I say which one, she asks when.
But if I'm more specific ("Remind me to call John Appleseed"), she just creates the reminder without asking when.