I came up with this idea last night... Joseph wrote about swords of obedience in his memo. He said he used some of them on ghosts he encountered... It seems logical to me that some of the ghosts doesn't appear as Joseph pinned them down before...

It's more than likely one thing: game mechanics.
It would be nice to face ALL the major ghosts, yeah, but "Team Silent" thought otherwise.

As for what you thought, the thing with Joseph and the swords, yeah that is probably correct. You find one of the ghosts pinned in the apartment world, afterall... You know, inside the room with the birthday cake? That ghost might be one of the first victims of Walter: the bartender.

No, the sword theory can't be correct, since there's only five and all of them can be found in the game.

I believe all of the ghosts you encounter are Sullivans, They just aren't all "Bosses" because lets be honest bosses are not what SH is really about.

If it was about bosses then Homecoming would have been the best in the series but SH is not Dark Souls.

If I remember right, one of the ghosts ("Old Victim") is supposed to be a placeholder ghost for the Ten Hearts as much as he's representative of one particular ghost.

It would've been cool if they had a model for each ghost, but maybe it'd have been a bit much (more ghost models means more pressure to use them?) to shove on the disc?

Even though the old ghosts all behave all pretty much the same way, all of them have different amounts of health, and have different sound effects. If you try and use the bug spray in each of them, they will each have different scream. It can be kinda funny sometimes.

More ghosts would be just a waste of time, I think.

Still, there's bosses in every SH game made by Team Silent. But I see your point, in Homecoming the bosses were really huge.

Yeah they were pretty good in that game, one of the best things about it, Scarlett was inspired.

But Cynthia (although quite generic for Japanese horror) was also quit brilliant.

And if it WAS Dark Souls, then the bosses in SH Homecoming would still be pretty lame. No SH game ever has what I would call good boss fights, but they can be good for Silent Hill. The boss fights at the end of SH2 kind of suck as boss fights, but thematically they're awesome.

I think having some ghosts as bosses and some as normal enemies was a wise decision. And having one "boss" ghost per area is a nice rhythm.