I must beg to differ with my earlier editor compatriot who said "readers have hated cover numbers since the beginning." Readers at the level of "Long-time editor" may consider them a joke, and while I agree with "former copy editor" that it IS lazy headline writing, magazines (over)use them because they work.
One reason I know is that I'm a guy with a growing gut, and those Men's Health headlines that say "[insert number here] New Ways to Get Rid Your Pot Belly Fast!" sucker me into buying that issue nearly every time.