It is unfortunate that we need to artificially require any applet that we put in the default panel config. It would be better if the panel treated applets like launchers in that respect: if an applet is in the default config, and the binary is not there, silently skip it.
Suggesting this as a F8 Blocker, especially because it is very bad that tomboy is required by gnome-panel.
(NOTE: This is not contesting tomboy by default. This is just to make it possible to remove tomboy and mono. tomboy can be listed in comps thereafter.)
Fixed in todays panel build