I've used MANY different types of nut materials. Yes there are some that I haven't tried. As far as the "integrity" of the guitar is concerned, none of them have an affect on it.
As far as "downgrade" is concerned, that is absolutely ludicrous, ridiculous, nonsensical, and downright incorrect. Bone is generally an UPGRADE from the OEM. After over 60 years of experience playing guitars and building them, my personal experience has shown me that some materials are better than others and bone is at the top of the list. IE: bone is an upgrade. Others may be just as good but when considering all the criteria that are important to me, overall, none are better.
AlexR is absolutely correct about trying to reason with him...but "lost cause" is the best case scenario. This reminds me of a quotation I heard once...can't remember who said it, and this may not apply to any particular forum members, but it says..."Never try to argue with an idiot. They'll bring you down to their level then beat you with experience".
Oh, by the way. Maybe I'm an idiot for not knowing this, but could someone please explain to me what "a functional nut material" means. And then what would a non-functional material be? Wet newsprint? Bubblegum? Saliva soaked licorice?
(Dave, please don't cite me for this post. I'm not necessarily trying to be derogatory to any specific forum member, but just providing agreement and support to comments other forumites have expressed, and my own personal experience/opinions).