I think the majority of time they are unnecessary. But; when the sun hits the lens at just the right angle and all you can see is glare, a sun shade is priceless. I think for 99.9% of the time a short 2" shade will work. I think these 6"+ long shades do nothing more as far as glare then a 2" shade would, but the longer one might help with heat mirage coming off a barrel. Ultimately they are lite in weight so I dont see it being a hindrance on a rifle being used during bright light conditions.
Good luck, Tony