Back to topic.
POINT #1
The ultimate question is: "If staff can have PCs and there are dozens of non-staff PCs to play with, why would anyone focus deep, meaningful RP towards an NPC? Why not get a player to app this NPC as a PC?"
I think the answer ultimately lies in the assumption that if a player creates an NPC and requests staff to RP as the NPC, then the player may have some element of control of how the NPC behaves.
"I created this NPC to be my character's bodyguard and sometimes lover. Can I get staff to run a scene with me involving this character?"
TEN MINUTES LATER
"The NPC I created would never do that!"
In this scenario, you literally get to tell people that they're playing the character wrong, and may be able to leverage not only the IC behavior of the NPC, but also predetermine the outcome.
POINT #2
If you've been in this hobby for a while, you all know the benefits of RPing with staff, staff alts, and in staff scenes. There are benefits, including immediate benefits(xp, equipment...) to role-playing with the person who can literally type in the commands to reward you for the scene or fulfill content requests without the player having to submit a job and wait.
So, I think it's fair to say that when everyone knows the point of these games is to create a PC and mingle with the other PCs, that the real trick to getting quick and preferable responses from staff is to put yourself at the head of the pack by investing one or more staffers in your content. I think this concept goes against the spirit of these games and promotes favoritism under the guise of it being "just an NPC scene", and given the weirdly personal implications of TS that MSB never really talks about, TS with a specific staffer playing sockepuppet of said NPC is shaky territory.
The preferable approach is to role play your own NPCs as puppets who don't affect plot and if the NPC deserves a bigger role, try to find someone to play them.
And once again I'll reiterate that my opinion is that the difference between a STAFF NPC and an NPC should be no difference at all. When a staffer plays, they are off the clock. It's a level of "conflict of interest" that deserves more attention, IMO.