I dont think the value of start-ups with a design co-founder gives us much more value than saying, designers are very beneficial for a product-oriented startup.
Im sure we can have similar calculations with value of startups with a single founder, with a business co-founder, startups that have their hackdays on friday ...
I don't want to marginalize design co-founders, I am looking for one for myself, but I dont like these sorts of conclusions and statistics