Well I don't know about octane mixing and all... if you want an in-depth lecture on it, ask Rich :D
But obviously, if you mix high octane fuel with a lower octane fuel, you won't end up with the equivalent as the high octane, because you're starting to screw with your ratios..
You will end up with gas has a higher octane than the pump gas, but a lower octane than the race gas. Extactly where you break even is going to matter on your mix ratio...
I say if you want to mix half and half, go right ahead... you'll end up with something better than pump, and slightly lower than race... it's not going to hurt anything.
Senior KXRider, I would definetly agree it's a heck of alot easier to keep your bike running consistently if you do one or the other. When you start mixing things, you are changing variables, and that will mess with the consistency of your engine. If you're not super concerned about your "performance" I wouldn't think this should be a huge issue, although I have heard that straight race gas is much better for your engine...
(My advice is worth what you pay for it though :p)