Well, they aren't stronger. The barbell deflecting more doesn't add any longevity to the bar (not sure how this would happen?). Basically they help you lift more weight.
You might want to read this thread, starting at Post #9: http://startingstrength.com/resource...ml#post1200035
Savs estimated that you get 10# out of a DL bar (plus bumpers) at 405 and 30# at 700: http://startingstrength.com/resource...ml#post1201096