I've found that this behavior depends on where the cursor is. If in the shared screen, the command goes through to the server, if outside, it applies to the local Mac client.

and isn't this how one expects the thing to work?

Exactly.

Do you mean to say that it depends on whether Screen Sharing is the active (frontmost) app on your local machine? I can't imagine that they would set it up to respond to which window the mouse pointer is over (and indeed that isn't how it works on my machine) since there are actually two mouse pointers whenever you move it outside the screen sharing window (one local and one remote) and that would become terribly confusing (and therefore un-Apple-like).

As for the original hint, I have to say this seems like a bad idea to me, and a bit extreme just to get around a minor annoyance. Setting the defaults value was fairly safe, since some programmer had to anticipate its existence, or it wouldn't be coded into the app in the first place. Randomly killing a spawned process that the app may be relying on for more than you think is not something that a programmer should be expected to anticipate or accommodate. At the very least, this hint should have a very prominent "Use at your own risk" warning...

I'd prefer that DoNotSendSystemKeys were still supported. Or, better than that, Apple should include a menu item and a toolbar shortcut/button option. But in the interim…

• Killing the RFBEventHelperd daemon seems to work regardless of the DoNotSendSystemKeys setting.
• This is a one-shot solution; need to redo it every time you invoke a screen sharing session (not surprising).
• After you kill the initial RFBEventHelperd daemon, Screen Sharing will startup another one whenever it's moved to the foreground, however the second one doesn't interfere with command-tab, and it will quickly (but not immediately) die if Screen Sharing moves to the background.

try
	do shell script "$(ps -cxao pid,command | grep RFBEventHelperd | awk '{print \"kill \" $1}')"¬
	    with administrator privileges
	activate
	display dialog "This session of Screen Sharing should now ignore Command-Tab." & return ¬
	    & return & "You’ll need to rerun this script for the next Screen Sharing session."¬
	    with icon note buttons {"Okay"} default button 1
end try

I used the name "Kill shared Command-Tabs", but you may call it whatever you like.

Note: If you don't want to display the dialog at the end of script execution, you can reduce the Applescript to this:

do shell script "$(ps -cxao pid,command | grep RFBEventHelperd | awk '{print \"kill \" $1}')"¬
	    with administrator privileges

Theory of operation: The do shell script calls a bash (i.e. Unix) script which looks for the RFBEventHelperd process. The awk command extracts the process ID, and creates a Unix kill command, which is then executed as root (necessary in order to kill RFBEventHelperd, as it is owned by another user… i.e.: _ard).