That's a good point--I didn't think about it from that angle because I have never set up the custom vibrations on my iPhone. I have to believe that's coming to the Watch too though--especially because the taptic engine is one of the main marketing pillars.
I hope you're right. It's obviously doable on the watch - with the taps and the vibrations (aka prominent haptic), the watch could simply mimic the settings on the iphone.