There's flare and there's excessive flare. All optical systems have flare. We just don't notice it unless it's excessive. Normal shooting conditions have normal flare and flare is part of normal film processing and part of film speed.So I actually disagree with the notion effective speed is increased by flare effects. Perhaps the speed "point" is increased, but local contrast in the shadows is decreased. If speed is a means to an end (ie sufficient local contrast in the shadows), we've lost more than we've gained, and this is compounded by reducing exposure.
Fun fact about flare and shadow compression. Ever wonder why Tri-X professional, with it's long toe, is considered a "studio" film? There's less flare with interiors and the ability to control lighting. In theory, regular Tri-X shot in daylight would have similar tonal distribution in the toe as Tri-X Pro shot in the studio.