[Confusion]

It's not about having, or being able to develop 'marketing skills'. That's too shallow a sketch of all the tasks a non-technical founder fulfills. I'm an employee of two technical founders that have developed all the skills necessary, but if I see which they are and which tasks they fulfill (all kinds of business administration, HRM, legal, PR, sales, marketing, company strategy and planning, procurement, CRM, etc.) then I would be glad to be able to let a future non-technical co-founder that actually likes a portion of that stuff deal with that. They hardly have time to code anymore :(.

So, in short: a non-technical co-founder fulfills many tasks and likes them. A technical founder would be smart to associate with a non-technical founder, so both can do what they're good at.