10. Sometimes (perhaps rarely) a better idea comes from a group.
9. There really is a lot of work to go around.
8. If one of the founders is willing to sell, take more investment or worse terms, perhaps they can apply pressure to the ones who are reluctant.
7. Most humans like to be around other humans, so cooperation is the norm among people who self-group.
6. Expertise can be complementary and allow a natural division of labor in the early days.
5. Hard feelings are more evenly distributed as the founders get pushed out or diluted one-by-one; push out the weakest contributors and least resistant first.
4. More founders = reduced risk and decreased dependency on one possibly loose cannon
3. If the board stops getting along with one founder, perhaps another will be a better fit. (see: Twitter)
2. Multiple complementary founders are rarely of one mind, and therefore it's easy to split control.
.. and the number one reason why VC's like multiple founders:
1. Extra founders cost them nothing