Has anyone ever used it in their campaign? I've never seen it actually used. Nor have I seen it in any of the books I have read other than it being a secret language amongst Druids (Which I have never seen more than one at a time) If you did use the language, what did you use the Druidic language for?
it was never a plot-event, but I played a druid once, and discussed my personal mission with my elder once, so I was sure the group didn't hear it.
It was more of a social thing, that I speak druidic with druids.
Also it might be useful to have a universal language, if you travel far, don't know many languages, you can still speak in druidic.
It might even make a cool plot element for a scoundrel adventure, where the group needs to convince a druid to tranlate a message, even if that would cause harm to fellow druids.