What about Larry Page and Sergey Brin? Or Steve Jobs and Steve Wozniak?

Motivation, more workers, and new ideas are all valuable contributions, but I feel like you might have missed the biggest benefit of all.

Co-founders can be good at the things you, personally, can't do. This is most visible in the business/technical co-founder division, but it can generalize to all kinds of tasks. If you're terrific at engineering large-scale filesystems (or whatever), but you can't build a web interface (or whatever), you might really need a technical co-founder with a different skill set. Every day or so, my co-founder cranks out something that would have taken me a week or more in a few hours.

Don't underestimate the value of teaming up with someone who's as good as you are at different things!