First ... no harm to the engine. however, even "low lead" AV gas has really high levels of lead (which really enhances the motor octane rating) and as you see, the motor loves the octane! Also, AV fuel places a lot of standards on repeatability, stability and wide range of performance so you'll get consistent quality.
That all said the "cons" are:
- Well, it's illegal to run in a road going vehicle, just because of the road taxes and such. The Feds don't like you skipping paying for the hand ... all your bases are belong to us!
- The lead can kill the O2 sensor pretty quickly. In fact, often you'll see a beautiful shiny coating on it. It DEFINITELY kills the catalytic converter (if you have it on).
- It used to cost a lot more than Unleaded Race Gas, but that may not be true any more.