sheeblast, few reasons actually.
The only reason to run octane fuel is to stave off pre-ignition. It doesn't produce any more power by itself, only allows you to produce more power without detonation tearing your engine apart. There is no advantage to running any more avgas than necessary to keep it from knocking.
tetraethyl lead. Think about that last part. Head about lead pipe sweating? Lead paint? Lead is a poison, simple as that. There is a reason it was phased out of everyday use despite the fact it was terribly good at what it did... lead was an excellent paint base but it gave children brain defects. Lead is an excellent top end lube in fuel but we don't use it anymore because tail pipes were spewing it all over everywhere. It's bad enough the blasters burn up 4 oz of oil per gallon of gasoline, so they really need to spit lead out of the tail pipe too?
Avgas is expensive. Most planes measure fuel consumption by the pound, not gallon. Gasoline weighs about 7 lbs per gallon and planes burn it up at speed. They don't worry about the cost because they can't do anything about it. It takes a certain kind of fuel to run, a certain amount of fuel to fly, and a whole bunch of money to fill up. Nothing they can do about those figures. But why would you want to burn any more of the expensive fuel than necessary?
Let's say you need 95 octane (R+M/2) to keep it from detonating. Obviously 93 pump isn't going to cut it... so you buy some 100LL to mix with it. You start out at 75% avgas 25% pump. You've got about 98 octane and no knocking but you're burning more of the expensive fuel than is necessary. If you begin cutting down the ratio of avgas to pump gas with each sucessive mixed tank until you find the point where your engine begins to exhibit pre-detonation, you're lowering the cost of each of those tanks AND not polluting anymore than you need to. Why would you not do that?
