The problem with lower octane number would be detonation and the horrible damage it can and will cause an engine, if sustained. Melted pistons with holes punched through their crowns is a sight to behold. Detonation doesn't do any good to bearings as well. Otherwise, using a good quality, clean fuel should be fine, even if it is of lower octane. Higher octane will allow advancing the ignition, which is important, mainly, in high rpm.
In the good old days, detonation used to pose much more of a problem, as a lot of the parameters were "built into the engine" and had a very limited range of fixed adjustment. it ran a single, simple 2D ignition curve that managed the degrees of advance as a function of RPM. That means that you had to tune an engine to perform best based on the worst case scenario and sacrifice other operating points. For example, the your max ignition advanced for a high revving, high compression engine was based on performance in the upper rev range. This would sacrifice low and mid range power generation.
With today's ECU controlled fuel injection and ignition, there is much less of a problem, as the ECU can react to pinging (and other sensor data) and very quickly adjust parameters to avoid it (mainly by retarding ignition or changing fuel amounts or air/fuel ratio). The main issue would be that an engine designed to produce a certain power output on a high octane fuel will, most probably produce less.
In its new turbo charged gasoline engine, Mazda provides two power output numbers, one for regular fuel and one for premium. The number for regular is significantly lower than for premium. Torque, BTW, is the same for both. That said, Mazda allows use of any fuel with a minimum octane number (I think it is 83). That means that the engine management system has wide enough adjustment range to ensure safe and reliable operation across the entire range.
This long write-up in not condoning the use of lower octane fuel. I am also not fully aware of the design parameters and range of available adjustments of our specific Duke 790 engine. That said, I would venture to guess (and this is ONLY A GUESS) that using a lower octane fuel will not be damaging.
I ride because my wife
says it keeps me young and is better than if I get a girlfriend...