Originally Posted by happyvalley
The idea of "keeping up with traffic" means you are traffic and places oneself under the purview of just another class of motor vehicle subject to all DMV and DOT regs of any other motor vehicle on the road.
There are potentially as many reasons and rationalizations as there are individuals looking for loopholes on how to slip through the legal cracks sideways into two wheeled motorsport without paying the freight but LEOs and state legislatures aren't that stupid and eventually will come back to bite ya. IMO, keep it low power, keep it a bicycle with low power assist and it'll be a lot easier explaining why it shouldn't be regulated as just another motor vehicle.
Of course, riding motorbicycles at a leisurely pace and keeping to slower roads is preferable most of the time but sometimes you're forced to take a 30-40mph main road to get to your destination - in those cases, I personally think it's just far more respectful towards other motorists to keep along with the flow of traffic than to be a potential obstruction on the road.
You raise a good point in terms of the legislation of motorbicycles - but anything that was previously classified as a moped or motorcycle would likely see little change if motorbicycles were ever to be regulated (although they haven't been under much scrutiny for the past 100 years and aren't common enough for most people to fuss over). Seeing as the only vehicles not classified as a moped or motorcycle would be the slower motorbicycles you seem to prefer, it is not likely that they would fall under any harsher legislation than what mopeds currently fall under - for the benefit of complete peace of mind with the police with a large legal grey area patched.
In the meanwhile, talking to local police and getting quicker motor-bicycles registered as mopeds (adding a few harmless things like a turn signal set and wearing a helmet) seems to put riders completely off the hook with police anyway, which is not too much of a hassle for what prevention is worth.