Spoken like a DMU user. If you have 10 defenders then yea, you don't need ANY structures/traps to hold off zombie attacks. But for those of us not blessed with a DMU, sometimes you get caught with only 6, 4, or even 2 defenders. Then you definitely DO need your Wire Traps to slow down the Rudees (reanimated dead).
I do not own a DMU, but I've held off more than 1 infected attack with one survivor with a pistol and a vest.
It was a lvl 50 survivor with a kick-ass rare AE-50 and a rare shooting vest, but I did do it. and without traps.