A chatbox message will change nothing, if people can't read what is there now, what makes you think they'll read something else?
I see no reason why it wouldn't change anything.
Only if you don't do anything, will make 0 changes. ex. If you don't paint the wall, it will have same color, before you looked at it.
Mods will have one strong argument in their arsenal, to issue cb bans more easier.
"If you can't read what's directly in front of your nose every time you are on the cb, then you definitely deserve the ban."
However, it doesn't have to be a message about advertising. It can be anything that has more power over: Ask your questions in forum.
How are you today?
What's the name of that site....?
Um, are we supposed to create thread in forum guided by current cb message?
etc...
all those are questions, so, are we properly guided based on that message? Ofc not, so, message that is right now, have little to no meaning, it's not precise, and also, it's webmaster forum, where people seek for help. CB is fastest way to get any. Breaking rule in that message is necessary, to have any convo in cb.
So, my suggestion is to change that message to something other, more precise and productive, and more meaningful, doesn't have to be related to advertisement (though, it's the most annoying thing I could remember), but it should be sort of guide.
If you don't guide people properly, you can only expect chaos.
And if you treat little things with 'whatever' argument, that's what makes lots of little things, to mess.