young Ed
Veteran
a mirror, my god! have you ever thought of your aerodynamic profile and wind flows over you and your bike? 
joking aside, it is a bit more weight if you are that conscious of weight, it's another part to get damaged and is fairly likely to get damaged, i always do 'life saver checks' over my shoulder (i even do them when just getting out of a chair!) and i am always looking all around
whilst driving tractors on the roads and off road i never use mirrors because most of them are broken or folded back to avoid damage and i am always moving my head to check all round which allows to to see both the areas mirrors can see and those they can't. the only time they are really useful is when reversing a car or reversing larger vehicles (such as tractors) with a trailer especially in confined/tighter spaces
Cheers Ed

joking aside, it is a bit more weight if you are that conscious of weight, it's another part to get damaged and is fairly likely to get damaged, i always do 'life saver checks' over my shoulder (i even do them when just getting out of a chair!) and i am always looking all around
whilst driving tractors on the roads and off road i never use mirrors because most of them are broken or folded back to avoid damage and i am always moving my head to check all round which allows to to see both the areas mirrors can see and those they can't. the only time they are really useful is when reversing a car or reversing larger vehicles (such as tractors) with a trailer especially in confined/tighter spaces
Cheers Ed