After using the virtual keyboard for a while, my conclusion is that the only reason you would want an applet that enables and disables the virtual keyboard is to work around bugs where the virtual keyboard appears inappropriately, like Bug 440569.

However once that and similar bugs get fixed in KDE software, the applet becomes superfluous; you will want the virtual keyboard active all the time because it will be reliably appearing on-demand.

However there will still be two potential remaining annoyances with regards to the virtual keyboard
1. An app is buggy and focusing its text field with a touch does not make the virtual keyboard automatically as it should
2. The virtual keyboard correctly appeared automatically, but now you are done using it and it did not disappear on its own

For both of these cases, it would be useful for the System Tray applet to show and hide the virtual keyboard when pressed. I think this would be a more useful behavior for the applet.