Good question and a necessary one to ask.
I think marketing gets mixed up with design because design is an element of marketing but that does not make a designer an expert at all Internet Marketing strategy. Personally I’m an SEO and while I’m good enough with HTML, PHP and CSS - I couldn’t design something pretty to save my life. In my marketing efforts I can rank a site, I can analyze stats and uncover where the problems are and then I call in designers to fix the issues. Designers also make far better landing pages than many PPC professionals BUT that’s only designers who are educated in conversion optimization. Simply knowing design doesn’t make you a conversion expert any more than knowing how to tread water makes you an olympic diver.
Also - knowing design doesn’t make you the PPC expert in the first place.
Personally - I say leave each area to an expert in it and we’ll all fare better. I’d have wonderfully rankings sites that repulsed every visitor if I didn’t follow this advice. :)