One of the most powerful tools against contamination is pitching enough healthy yeast to quickly dominate the wort, and drop the ph to where most contaminants are unable to survive. If the fear is a few stray brett cells or pedio, etc... I think of it this way. If I were trying to get Brett or pedio to produce a noticeable effect, do I think a few airborne cells would be enough? No. Then you have theoretical vs reality. Are these guys who use foil for "open" fermenting experiencing contaminated beers. No.