It's not so much that plain gasoline will necessarily harm the engine, but that additives will help keep the engine cleaner and therefore should maintain/improve long-term performance.
All engines will have normal deposits, and it seems best to me to deal with deposits as you drive rather than let them get bad enough to cause an issue and then take the car in to have the fuel system cleaned.
I primarily buy Exxon but will occasionally buy other top brands; and have never had a single issue with injectors or fuel system on any vehicle. And some of them have exceeded 200,000 miles. I think it's cheap insurance.
I have on rare occasion purchased a tank of cheap gasoline, but have gone right back to Top Tier brands figuring the additional additives will clean any deposits over time.
According to the following it was the auto industry that requested higher quality gasoline (not to be confused with higher octane).
https://en.m.wikipedia.org/wiki/Top_...rgent_Gasoline
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