Me: "Who else do they think is going to use it?"
Rog Peek: "Any Tom, Dick or Harry that visits the site. As I said before, this function is available to members and non members alike."
But that's my entire point - you will have already found Mudcat, read the thread and possibly contributed towards it before you get to the point where you feel the need to share it. In other words, it's only Mudcat users who will ever use this function.
If the fear is that it will bring unwanted attention from 'non-members', I think this is unfounded. If I choose to share something, my 'folkie' friends on, say Twitter or Facebook may have already read it but if they haven't then I'd expect it to be something they'd be interested in either reading or contributing towards - why on earth would I share it with them otherwise? On the other hand, my 'non-folkie' friends wouldn't have the slightest interest and would instinctively ignore it. At best, they may read the thread and come away with a better understanding of that weird folk music that I'm always going on about (although this is optimistic given the nature of some threads). I don't imagine they'd ever feel the need to contribute towards any of the threads. I'm sure they have far better things to do.