This seems like a pretty common mistake among designers. As a matter of fact, Facebook made this same mistake themselves not too long ago. When they release the "cover photo" feature, they showed off how great it looked when applied to a bunch of professional designers' profiles. When normal people started uploading low-res, vertically oriented images, it was instantly clear how silly of an idea the cover photo is.

Facebook seems to acknowledge this since now if you view a person's profile, it automatically scrolls past the cover photo so you don't even see most of it.