I think what Kooty is getting at is ethanol is an alcohol…thus it aids in drying gas lines / seals out over time. Big discussion on this on the site. Unknown for sure but makes sense to me. Also, in those discussions the consensus was that ethanol was significantly less efficient in mileage. Third…it is cheaper.
I don’t know how it would be bad for you to go back and forth. 100% gasoline being the best option, I would think anytime you can put it in your engine would be a good thing.
Note…I am not a gasoline / ethanol expert… 
Ted