As the lens is plastic now I don't think they are of any benefit, I have not had a broken or chipped lens in years now had a few with glass lenses, and I did use the covers back then when the headlights started get very expensive.
If you have Xenon headlights it is now a requirement that they have washers to keep the lenses clean and avoid interruption / distortionto the beam. I am not sure if fitting lens covers would cause problems in that department. As Wakev says the lenses are already plastic so there may not be any benefit, especially if they made your car non compliant with the road regs.
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