If you replace a cookie for eviladcompany.com with one for eviladcompany.domain1.com, there is still an advantage: that this is not shared with eviladcompany.do

If you replace a cookie for eviladcompany.com with one for eviladcompany.domain1.com, there is still an advantage: that this is not shared with eviladcompany.domain2.com, so eviladcompany cannot know that it's the same person visiting both websites. Sure there are browser fingerprinting techniques, but at least you're making privacy invasion harder.