What functionality is missing? What is unsatisfying with the current situation?

I run a West-Marches style game (really a LARP, but this is the easiest way to think of it for normal workflows), where characters can be a member of several distinct groups all with their own competing side objectives. I'd like to utilize the existing party system for this, but at present a Character can only belong to one party.  

How does this feature request address the current situation?

Allowing a Character to belong to more than one Party allows things like organizational goals, quests and resources to be tracked independently of the characters, and lets characters belong to several such entities.  

What are other uses for this feature request?

Anyone that wants their RPG Characters to belong to multiple organizations of Player Characters (guilds, etc)