Interesting perspective, have you had experience with HUDs in arliners? I hear some Airbuses got them now.
While I do agree that cluttering your FOV is dangerous, if you HAVE to consult some sort of instruments, I'd imagine it's better to have them in your viewport and focused to infinity instead of having to look down. ..... - it's all a question of presentation IMHO.
Sorry for the long answer...
I've had limited exposure to HUDs. They are excellent for providing information immediately important to controlling the aircraft. However, I'll generalize on out to all of our flight instruments during complex operations - we don't look at all the data all the time. For a few seconds there is one most important piece of information. But once we verify it, another piece of information will become top priority. Some events are important to be notified of, which happens with either an aural alert (e.g. C chord for altitude) or a flashing icon on the display (e.g. capturing a course). Our visual awareness and mental processing of data is quite limited really, so I am relying on trends remaining what they have been and then sequentially checking different parameters to verify it. I cannot simultaneously attend to airspeed, altitude, vertical speed, lateral course (localizer), vertical course (glide slope), and position along the track.
During a complex procedure under busy conditions we will have our vision focused on the 2 screens directly in front of us, not out the windshield. A HUD simply displays this info while we are looking out the windshield. For the most part, we are either 100% focused on the instruments or we are 100% focused on the real world out the windshield. There is a minimal amount of cross checking between the 2, usually just related to one specific parameter. For example, once we transition to visual on an approach then the only information inside the cockpit of interest is the airspeed. A very quick glance down (the eyes know where to look) verifies it.
These operations take a lot of concentration and are fatiguing. You cannot fly all day like this.
Now when we're riding or driving, we are 99.9% interested in what is outside, not what our instruments are showing. We shift gears by sound and feel, not by looking at the tachometer. Once in a while we need to verify our speed. From a safety and operational perspective we have no need to have any information presented to us heads-up while riding/driving other than perhaps speed.
Very limited additional information could be helpful, but now we're into the realm of synthetic vision. In the real world, our technology will flood us with useless information. You may indeed not be sure which of the next 3 driveways to turn at, but your nav system isn't going to be silent except for that one piece of information. It is going to constantly bombard you with information. That's where I see the problem. People will be paying attention to what the GPS is painting on their HUD and not attending to the real world. And the vast majority of what is being painted on the HUD will be unnecessary.
Synthetic vision requires the system know precisely how your head and eyes are oriented in order to overlay onto your field of vision. Unlike a pilot who's head is in one precise position to use an aircraft HUD, a motorcyclist's head will be moving in all 3 dimensions. Motorcycle HUD systems will not be able to overlay a la synthetic vision at a price affordable by non-billionaires. The rider's eyes will have to move to view the HUD data, and will have to process it mentally. Perhaps it will be GPS instructions to turn ahead, or a notification that the speed limit is changing ahead, or traffic jam information. The rider will have to stop looking at the real world, and will have to use scarce brain resources to process the incoming HUD information.
Riders with HUDs will be no different than automobile drivers using screens in their cars. We've all seen distracted drivers looking at screens not at the road ahead. HUDs will turn motorcyclists into the same.