Interesting thoughts!! I definitely never paid attention to the Share button before making a rec. Poking around, it seems like it's a setting that can only be disabled at the account level, not work level? I wonder if people disable it mostly to avoid rapid propagation to Twitter/Tumblr, or if maybe they didn't really know what the checkbox did the first time they looked at the preferences and never went back later. It'd be interesting to find out the reasoning!