Yes they are, but you don't need to run with them. I always remove them from my Uno's as it helps to reduce brake fade, but i'm rather hard on brakes and go rallying
There are two schools of thinking:
a) shields are useful as they prevent dirt/water from getting to the disc while braking as every foreign object between the disc and the pads decreases the brake efficiency, and
b) get rid of them, you need to radiate the heat from your front brakes, as their efficiency is tangential to their temperature.
Honestly, I do not know