It should probably be noted that Reah sets up shop here at this altar once you rescue her from the Tomb of Giants. Probably isn't relevant to anything, but it's worth mentioning at the very least.
Back onto the topic. I really wish they did a slightly better job of expanding on the idea of the Fire Keepers.
Throughout the game, we encounter a total of 3 Fire Keepers (Anastacia, Quelaan, and the Darkmoon Knightess in Anor Londo).
We encounter a total of 4 Fire Keeper souls outside of those held by the still living Fire Keepers (one on the altar in the Parish, one on a lone platform in New Londo, one in the sewers/storm drain section of Blighttown, and one in the cells in the Duke's Archives which also happens to house Logan).
What I would really like to know is, who/where do these Fire Keeper souls belong.
In the game, we find out that killing a Fire Keeper deactivates the Bonfire that they are associated with. Killing Anastacia for example will shut off the Bonfire in Firelink Shrine.
That said, we encounter probably a good 30 or so Bonfires that we never meet the Fire Keeper associated with. Where exactly are all of them at? It stands to reason, using the examples of the known living Fire Keepers, that they must remain at least somewhat close to their associated Bonfire. So are we to just assume that all of these stray Fire Keepers are off hiding or just in inaccessible sections of the world around us?
I have heard mention that the Players Character is in fact a Fire Keeper himself, and that would go some way to explaining how all of these other Bonfires remain lit even with no Fire Keeper to be found, since you yourself are the Fire Keeper. It would also explain how the Player Character is able to light unlit Bonfires and his/her ability at the end to manipulate Gwyn's Bonfire.
This raises the question of why the Player Character is unable to relight the Bonfires associated with the still living Fire Keepers after they die though. If the Player Character has the ability to light and become the Fire Keeper for all of these other Bonfires, why would he/she be unable to take over the position for these slain Fire Keepers. This also brings up the question of if it is even possible to be a Fire Keeper for more than one Bonfire at a time.
Then there is the question of those 4 Fire Keeper souls we find throughout the game. There really is no mention of who they belong to, and it occurs to me that they must have been Fire Keepers associated with Bonfires that we never come across in game. Since these Fire Keepers are obviously dead, their Bonfires would be snuffed out as well, and, should they behave like the still living Fire Keepers, once they die, their Bonfire can not be relit by another. The only time we see one relit it is done by the Fire Keeper herself after you return her soul (and thus bring her back to life).
Since we never encounter any Bonfires that we can not light, I take this to mean that we never see the Bonefires associated with these Fire Keepers.
It should be noted also that all 4 of the found Fire Keepers souls are in locations that were once populated. The Parish, Blighttown, New Londo, and The Duke's Archives are all places that I am sure at one point probably had a decent population.
I realize my wall of text probably didn't do much to answer anything, but as usual, stuff to think about.