I've heard this argument before, but I don't buy it. All it really proves is that group think is alive and well.
Two classic counter-examples:
1. I can come up with the most ridiculous idea in the world and still find someone who I could convince of anything.
2. I can come up with the best idea in the world and still not be able to convince anyone to join me, but be able to convince many people to buy it.
Have you ever tried to convince someone to join a truly ridiculous idea? You'd be surprised. People aren't that stupid. We are talking about someone joining as a co-founder. Therefore, that person has some basic notion of what a co-founder is, what a startup looks like, etc. We are not talking about finding a random person on the street who'll give you a quote "great idea".
For the best idea in the world, I also challenge you not to find anyone who believes in it.
Now, to be practical rather than extreme, my experience shows me that having co-founders brings a lot of value during the brainstorming phase. Although I like to think of myself as very smart - I could figure out everything on my own, right? Co-founders bring balance. Once in a while, I have a crazy idea that deserves to be killed. But I'm crazy enough that on my own, I might do it. With co-founders, you get very quick reality check.
The secret is to find co-founders who share your vision 90% of the time, and help keep you on the right track the 10% when you are wrong.
If you don't share the vision with your co-founders and can't even agree 50% of the time, then you are correct: it will be a waste of your time, arguing endlessly about trivial stuff.
I think close to 100% is ideal, as long as noone is being a yes-man.
For whatever it's worth, I don't think that it makes that much difference to success; but I do think it makes a difference in your chances of getting funded. I don't see those two things as necessarily related.
I'm sure they don't - I mean, who's ever accused VCs of groupthink?