Well, the theory is that since higher octanes take longer to combust fully, using a higher than necessary octane might cause the fuel not to burn up fully and cause problems with emissions equipment. But it seems like if this were really an issue, you’d hear about it more because newer cars are pretty sensitive to emissions problems and there’s a lot of people out there who use higher octane gas than they need to.
I will also mention that if your car is turbocharged, the altitude effect on octane requirements is less pronounced, which is why the premium is still 91 octane at high altitudes.