Why would they be very useful? Can you give a practical use of this function?

I don't know about getting, but setting would be good. For example chatbox templates etc. setChatBoxInputText("say", "/me is going to") and then the player can complete his/her sentence

Getting could be used to steal user passwords as they try to /login via chat box.
And setting could be easily replaced with outputChatBox?

for example for PM's. I dont need any big ugly gui window which will take a lots of space. If player writes a pm to someone, it will automaticly fill in "/pm recievernick" everytime he opens chat input, until he deletes it.

"set" function is necessary, but yes, could work without "get" also

Yes getting it could be dangerous, if the input is found to be containing "/login" or "/register" then it returns false? What about getting and setting console input?

It could be useful for autocompletition. And if you want to steal user pw's just create a gui with username and password and grab it...

hax = guiCreateEdit("password",0,0,1,1)
addEventHandler("onClientGUIAccept", root, function(g) if g == hax then
server.outputChatBox(getPlayerName(getLocalPlayer())..": "..guiGetText(hax) end
end
)

I like this suggestion as it would be very useful :).

Here's another reason: I'm currently doing a custom chat-input, by overlaying the default input with my own. Like this I can keep the ESC button to just escape fron the textinput. Now, the issue is that my chat overlay will almost certainly have another input than the default one. Some other stuff around, command related, I only want to keep the text from my input. So if the have the SET function, I can keep the default chat input empty and completely manage my own input.

I'd say, dont use GET, because this could break security. You can easily retrieve someones password when typong /login with this function.

I hopw we can see the SET function in 1.3, because I would really like to finish my own chatinput overlay.