It isn't a gimmick. The sun can be damaging to dogs' eyes over time. It has been proven that limiting the exposure to sunlight slows the formation of canine cataracts, too.
It is encouraged that pet owners take their dogs for their walks either early in the morning or early in the evening, or both...but not during the part of the day where the sunlight is the strongest.
Some canine cataracts are caused by other things, and lifestyle is a factor, but shielding your dogs eyes actually is recommended if they are out in the sun for long periods of time, especially the sunlight that occurs from late morning to early afternoon. =]
Ours aren't outside very often at all...and they don't wear visors or sunglasses because of that.