BTW, I'm saying this as someone who is going it alone, even though it makes things riskier.
It's all in the details. Let's say we have the same success rate for single founder and multiple founders ventures. There are many more multiple founders startups for at least two reasons: (1) it takes some guts to go alone and (2) VC/angels won't fund solo undertakings. Since the multiple founders pool is bigger, there are more successfull startups of this kind. Here you've got your misleading statistics.
Like the silly age debate, I guess it all comes down to this: do what you can with what you have. I certainly think that there is some 'cargo cult co-founder searching' going on around here, meaning that you shouldn't just grab a warm body because PG said so, but you should already know that person by the time you're ready to go. And if there isn't a someone, well, tough, do your best in any case... that's where I'm at.