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In my opinion, settings about displaying plasmoids on systemtray should be Activities aware.

For example on entertainment activity should be always displayed media player controls, NetworkManager plasmoid, etc.
On communication Activity should be always displayed NetworkManager plasmoid and kopete's tray icon.

Or disable those systray applets, and in each Activity add a standalone widget for network-manager, media control, and one from kde-telepathy (There does not appear to be one for Kopete as far as I can tell)

That only works for Plasma widgets that are (for convenience) added to the System Tray. However that doesn't work with the many apps which use the status indicator or embed or whatever protocol to place an icon in the system tray.

Besides, to me it just seems simpler for the System tray to be Activtiy-aware and allow you to show/hide/auto different entries on a per-Activity basis.