They're practically useless as has bee stated above. Bike manufacturers put them on to cover their a$$ in case of a law suit.
Aside from the practical and logical reasons not to use them, they're not cool, and many many roadies are heavy into looking cool while riding. Including meself.
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See, this is why we can't have nice things. - - smarkinson
Where else but the internet can a bunch of cyclists go and be the tough guy? - - jdon