Here in Portland the gas stations are required to sell 10% during the winter to help with maintaining our air quality by the DEQ (Dept. of Environmental Quality), I think they may sell it all year round now (I quit paying attention to it after my car ran fine after a winter's worth of the stuff).
I think what the mechanic was trying to say was that E. eats up the rubber seals in the carbs if you let it sit in the carb too long, and therefore there is nothing he can do about it and won't warranty the carbs any longer.
I work in the Tech Support Dept. for the Kroger Corp. our gas stations have a "black box" in the back that constantly monitors the holding tanks for pressure drops in the lines (leaks) and water in the tanks, if the water level gets too high it trips an alarm and shuts down all the pumps, at that point we have to dispatch a field tech to fix the problem.
Our pumps also have fuel filters built into the individual pumps that should stop any crud from getting into your gas tanks.