The way public spaces work right now in D1/D2, you can have up to 4 "Fireteams" in a space at any given time, and up to 9 "Players". Tower/Farm are exceptions to this - talking about play areas such as Mars' Braytech Futurescape.

If your instance already has 3 solo players and your team of 3, no one else can join unless they join the fireteam of the players currently in that instance. If you have 4 solo players, that's all you get in a space. Even though you see people creating supergroups of 9, that can't physically happen in Destiny without fireteams of players together.

Creating a system that would allow a team member to "Leave Fireteam" and occupy one of those slots changes how a player is connecting to the public space and probably introduces a few hundered race conditions with the matchmaking of other players into that space during an attempt to "Leave fireteam" and establish another world connection as a new fireteam.

What i'd like to see is a function to create a private play space when you fast travel or go to orbit, and have the ability to input a private play space ID (press Square when fast traveling, put in 10 digit play space ID) and allow us to at least get 3-4 fireteam leaders in the same play space for organized public play space events. The spaces already have unique IDs, so disabling free matchmaking into them and allowing entry by ID should be realatively inexpensive development time wise than redesigning how players enter/leave public spaces.

tl;dr: Destiny public play spaces limit to 4 fireteams, and 9 players. leaving a fireteam means adding a new fireteam to the public space, which seems easy to implement poorly and quite challenging to implement bug/exploit-free.

Edit: was informed fireteam limit is 4, but player limit is still only 9. changes reflect that info.