I have one friend who is a "business guy" but I wouldn't start a business without him. His value is priceless. Admittedly he won't be building the product with you but the contacts he makes, the meetings he manages to arrange and the time spent getting other people excited about the product make it a lot more enjoyable to work on since you know there are others out there just as excited as you.
The real "business guys" are the ones who shield you from the outside world so you can focus on the technology and can find and make the connections with others that you're too busy hacking to do. Unfortunately these people are often grouped in with the "idea guys" you all to often see online saying "I have a great idea, build it for me and we'll split the profits 50/50" but then don't actually contribute anything to the process besides the original idea.
Yaw, FWIW I was going for a thesis of "For all the people pouring hate onto the biz guys, it's a bit more complex than that".
Regarding the "shielding from the outside world" idea: For a pre-funding startup, I don't think there's much outside world to shield you from-- certainly not 10-12 hours a day of it. There are some products that can benefit from sales effort on day 1. Most (especially consumer plays) really can't. For the first 3-4 months, what is a biz guy gonna do, other than sell?
I should also add that I truly GET the value of a biz guy. Every company I have build has required that evolve (or devolve, if you want to be a hater!) from builder to biz guy. I've played the biz role for more of my adult life than I've played a development role.
"There are some products that can benefit from sales effort on day 1. Most (especially consumer plays) really can't."
Admittedly selling(to customers) isn't always needed on day one but having someone working on building relationships with other companies and arranging cross promotion, etc is definitely valuable no matter what stage you are at.
If you have a demo or a UI prototype, even just a click-through, someone (other than the designer) should be running tests with users.
If you're planning on outside funding, someone should be working on that well before you need it. (You start raising round N before you close round N-1.)
The real "business guys" are the ones who shield you from the outside world so you can focus on the technology and can find and make the connections with others that you're too busy hacking to do. Unfortunately these people are often grouped in with the "idea guys" you all to often see online saying "I have a great idea, build it for me and we'll split the profits 50/50" but then don't actually contribute anything to the process besides the original idea.
Business guys good, Idea guys pointless.