This may actually become a security loophole, and as another user already said that you may not necessarily be the one actually wearing the watch. If the concern is typing a passcode, you'd still need to do this on the watch regardless. The only time you don't need to type your passcode on the watch is if your iPhone is already unlocked, and you have the Watch set to be unlocked by the iPhone. This being said, you'd now see a loop on which device will unlock which device.
For now, the reason why unlock your mac with the watch "somehow" makes sense is because the center of your authentication is still the phone. If for example, I am not wearing the watch and I try to bring it close to my locked Mac, the watch would ask me for a passcode, before it will unlock the Mac. My watch will be able to unlock my Mac if before the mac was locked, I was already wearing my watch, and the wastch has already been unlocked by my phone through the API. If not, I still need to enter my passcode on the watch. So in the end, while it "may" be more convenient to unlock with the 4 digit code on the watch, it poses a potentially less secure process flow.