Is it 100% necessary? No. But it does reduce friction on the front brake cable, which will improve brake performance. If you have a lot of spacers between your stem and the cable hanger you should be OK. If they are close, then you may want to add it. As long as you didn't trim the brake cable too short it should be pretty easy to take it apart add that piece (just trim the housing if there is too much left). A new brake cable is pretty cheap as well, might not be a bad idea to replace it and get it done right.
PS... If the bike has canti brakes and all the cables run down the top of the top tube, it's a cyclocross bike, not a road bike.