Aren't they both content management?

If I found Robert Silverberg because I love science fiction, and then read "Book of Skulls" which is anything but, that doesn't mean I've transferred my love of science fiction into a love of Robert Silverberg. Likewise, "books about monastic orders" would include "The Name of the Rose" as well as "Canticle for Liebowitz" and "The Book of Skulls" which gives you three different authors - I can follow Umberto Eco and Robert Silverberg without worrying about how they're connected or what impacts they have on each other. Likewise, following UmbertoEco is likely to lead me to Carl Hiassen or Milan Kundera much quicker than #MonasticOrder is. Meanwhile, #Roadtrip would include "Book of Skulls" but neither of those authors and neither of their books - but it would lead me to "The Road" and "Zen and the art of Motorcycle Maintenance" where I can accept or reject Robert Persig and Cormac McCarthy on their merits, not their relatedness. Should I decide to follow Cormac McCarthy, it's going to lead me different places than if I follow Robert Persig, and that's appropriate. The two functions are intertwined, they do not compete.

I think you see it as an "either/or" proposition while I see it as an "X/Y" proposition.

As far as "finding their own uses" I think you'll discover that those uses have been found. "Tagging" is not new, is widely accepted, and has a pretty broad featureset that has been adopted across many industries. It just seems to me that the uses your users have found don't entirely jive with the ones you want them to find. ;-)