I have run an R22 on MOGAS/AVGAS mix to get me home & I agree with most previous comments re valve lubrication/octane rating etc. but an important point has not yet been mentioned - AVGAS has additives which makes it more resistant to vaporisation. Vaporisation can be caused by negative pressure in the fuel line (eg fuel pump suction lines) or overheated fuel (a lot of fuel injected engines are tricky to start when they are hot due to vaporisation in the fuel injection lines running past the cylinder heads).
The STC kit for the R22 addresses this by providing a ducting around the fuel bowl which directs cool air over the bowl to prevent the fuel from becoming too hot via heat from the engine bay. If the fuel did vaporise in the bowl the engine could miss or even stop which is generally not desirable & would usually occur when you had a high power demand from the engine.
Personally I believe the probability of vaporisation in a gravity fed system is pretty low but they obviously need to cover there bases. In my case I knew I wouldn't make it to the next available AVGAS & so I elected to land early & top up with MOGAS while I still had plenty of AVGAS in the tanks to mix it with.