Otherwise, it confuses users when they change notification settings but do not see them applied (this happens if another notification daemon is running - notification preview is showing but isn't affected by the settings).
This is a bit of a tricky issue - at least as long as xfce4-notifyd exits after 10mins of inactivity. If we remove this "feature" I guess checking for a running notification service is a valid approach.
Ah, yeah, right - the "feature". But you've been planning on getting rid of it anyway, right? Another approach could be trying to start xfce4-notifyd on opening the dialog and showing a warning if start fails. This would work along with the "feature".
The "feature" has now been removed in master so feel free to implement a check if you have time!
Simon Steinbeiss referenced this bugreport in commit a0e8487336d9c9eb54548a73f8d17a13267fa8fe Show warning in settings dialog if notifyd is not running (Bug #13417) https://git.xfce.org/apps/xfce4-notifyd/commit?id=a0e8487336d9c9eb54548a73f8d17a13267fa8fe
I guess the sanest way to implement this is by checking via dbus (see the commit above) - I wonder though if that will work as it may not distinguish two notification services that share the same name.
I consider the issue fixed in 0.4.0