I think it's more for the sportbike guys that have there plates tucked under the rear fender close to the swing arm mount. I've seen bikes with the plate mount flat under the fender and the only way to see it, is if you look up under the fender.
Most of the guys that have their plates mounted this way are hooligans. The guys that are endangering their lives, and yours, by splitting lanes doing 100 mph. Doing endos at stoplights, it's real nice to see that in a mirror, bad enough in a car, even worse on a bike.. Riding with one wheel in the air while splitting lanes. Or weaving on and out of traffic at excessive speeds....
As for mounting a plate vertically as is done with many customs, oh well... You don't have to change 'em, just be prepared for the ticket. But more than likely a custom built bike, that's registered as a custom build bike, will be grandfathered. If you changed your plate from horizonital to vertical on your bike that has a manufacture's VIN (HD, Honda, MV Augusta or whatever), you will probably get a ticket.
You probably won't see local PD out "hassling" most riders. You probably won't see the Daytona PD stopping most people during Bike Week. But it is another way for them to stop you legally. Which will lead, I'm sure, to a check of ID, VIN, If you are legal to ride without a helmet per state requirements, a check to see if you have any wants or warrants, etc.....
