In my experience, both as a role-player and as a staff member observing the forums, you are more likely to find that the 'right partner' for one plot is not the same as the 'right partner' for another plot. Everyone has different interests and strengths. Some people don't like group settings, some people thrive in group settings. Sometimes the person who likes vampire settings can't stand modern-day settings.
In addition, if you don't limit yourself to trying to find one person that fits all those roles, you are less likely to have your role-playing experience tied to another person's schedule. People take breaks, have real life issues, and sometimes even lose the creative spark for a time. The chances of having all your stories stopped for any of these reasons diminishes when you have multiple writing partners.
