Personally I do not think that the transition lenses can replace sunglasses, since it is not a substitute for sunglasses. Outdoors, Transitions lenses automatically darken depending upon the amount of UV radiation present. The brighter the sun, the darker Transitions lenses get. So, they help enhance the quality of your vision by reducing the sun's glare in different light conditions – on bright sunny days, on cloudy days and everything in between.
Transitions lenses are a great everyday solution for outdoor use, but there will always be times when a second pair of sunglasses is beneficial. For instance, since extremely high temperatures can affect lens performance, Transitions lenses may not always get as dark as sunglasses. Also, since they are activated by UV exposure, most Transitions lenses don't work behind the windshield of a car (your windshield actually blocks UV rays). So sunglasses are still necessary for extended time in the sun and various activities.