In distributions like Debian, people are likely to install new panel applets and remove panel applets previously installed. This is quite likely to occur while the panel is running. At least periodically, and perhaps every time the menu is opened, the panel ought to scan the plugins directory and update its internal list of available plugins.
I'm not sure it is worth the effort. I will be trying to implement loading on demand / unloading when no longer used, so perhaps I can incorporate this idea. However, installing / uninstalling plugins is not part of the normal operation, so I don't think it is wrong to require a little user intervention here. The panel will get a 'Restart' menu option, that will update the plugins and configuration. Until that time, just use 'pkill -USR1 xfce4-panel' to restart the panel.
A 'reload' or 'rescan' or 'restart' would do the trick. People are used to the menus auto-updating from things like gnome. Once the session manager is in place so that if you kill the panel you get a new one launched automatically we'll be somewhere safer with respect to not noticing changes until the panel is restarted. On demand loading/unloading sounds good, and I think you could incorporate scanning the plugin dir on demand as part of it quite sensibly.
Jasper: Once the session manager is in place there won't be any problems restarting to panel on demand. I think we'd better wait for Benny to finish his work instead of duplicating it
Ok, I'm closing this now on accounts that it is a) not a bug and b) CV has a Restart menu item that will accomplish reloading easily.