There are a lot of scenarios where it could come in handy. If you have an npc that will only let you pass if you answer a riddle or 
solve a puzzle, it would allow a player to choose an answer without giving their answer away to others. However, I think the 
most important use would be for creating unique ways to interact with npcs. I have experimented with a lot of minigames 
using npcs. All of them involve saylinks being used as buttons/controls essentially. When you click the saylink, it gives the 
npc a command. It would be much better to have the player not actually say the command and just have the npc react to 
it as it would cause less spam. Here are some examples of minigames I was using them for:
The first one is a text based adventure/rpg game that a group of players can play together with one npc:
If the player actually said what they clicked (Move North, Attack, etc), it would cause a lot of spam to other players - 
especially if there were multiple games going on near each other.
The next one is basically a breakout clone:
This one is an idea for a custom lockpicking system based on the game Mastermind - The player has a certain number of 
tries to guess the combination of ones and zeroes that will unlock the door. Each time they make a wrong guess, it shows 
them how many of the digits were in the right place:
Another important feature of private saylinks, combined with popup windows and proximity say is being able to make custom 
player commands that are parsed in a script rather than the actual source code. Here is one hypothetical example of a menu 
that a player could bring up by clicking an item or something:
Hypothetically, the player would click one of the saylink menu options from the popup window and it would cause the player 
to say a command privately (like .banker or .CastSoW etc) and you could have an npc that is responsible for handling all 
player commands for the zone. The npc would be set up to use proximity says and would have a proximity that reaches the 
entire zone. Whenever the player clicks a command saylink, the npc would react as if the player said that command and do 
whatever the command is supposed to do.