Plastic frames hide the lens thickness and support the weight of glass lenses. They require more care in the choosing stage as they are less easy to shape. Personally, I go for optical quality over pure vanity and plastic frames allow me to have large area, low aberration lenses. Having said that, they have to look acceptable and I always had a problem with thick lenses in metal frames looking like glass bottle bottoms. Plastic frames+glass comfortably hide a 4-5mm edge thickness. As for the strength the pairs I have now have metal running through the arms.