To me, it looks like the customers do not know the difference.  Also, designers have an eye for beauty while most developers are not very gifted in color selection and other aesthetics, so on the surface, for those who do not know any better, designers appear to produce prettier products.  They can create the eye candy most developers do not pay attention to. 

One danger of hiring a designer is that if they run into a bump, they either resort to plug-ins that may slow a website down and may not be properly maintained... then a year later a developer finds such plugin is unsafe but it's at the center of the website and removing it would distort all pages;

Another problem with hiring a designer is that a designer may not have enough knowledge on how to protect a client's website when hiring a subcontractor to do the changes they cannot do.  So they potentially put the client website at risk.

I recognize graphic designers have the aesthetic abilities to create beautiful concepts, that's why I have a graphic designer create the concepts and a developer finish the job.