Version:            (using KDE 4.2.2)
Installed from:    Ubuntu Packages

In the default panel setup, the taskbar swallows all the available space, and the system tray is sized just big enough to fit all of the system tray icons.

If you remove the taskbar, the system tray expands to absorb the extra space.  I believe this is undesirable.

a) It conflicts with the behavour of other default elements, namely the Device Notifier and Network Manager widgets.  These widgets are similar in intent to certain system tray icons.  Often, we want all these icons together. and having lots of empty space inside the system tray causes the icons to be separated.

b) If you want to put the system tray on a panel without a task bar - maybe you fill it with application launchers - you may wish to use a spacer [1] to separate the system tray from the rest of the panel contents.  I submit that this will be easier/simpler for users to realize/discover if the system tray does _not_ expand to fill the panel.  Otherwise, users may be put off.  Without trying, it may not be obvious that the spacer has a higher priority, and will eat _all_ the available space, as opposed to splitting the available space between the spacer and the system tray.

[1] http://www.notmart.org/index.php/Software/Make_some_space