I think it also protects against people injecting content into the stream such as with the (possibly acryphorial, but IMO hilarious) goatse attack at Defcon 12[0]. Yes, people can still impersonate the AP, but that attack takes a little more effort and hardware.

0: https://emaknyus.blogspot.com/2011/07/airpwn-bringing-goatse... (warning, likely contains goatse - image did not load for me)