I don't get why "idea people" or people who can't contribute much to the launch of a product pick up some design skills to at least help convey the ideas they want implemented.

I'd only pick a cofounder that doesn't do engineering if they have deep connections, domain expertise in a relatively complex field that I myself can't pick up easily or has alot of red tape, etc. This person also needs a public past track record of success too. It'd also depend on the industry too, b2b is where this partnership has more merits (and larger contracts), b2c not so much since the barrier to entry is lower