A CAPCHA would sort the wheat from the chaff for those that choose to use it.. Even if only as a tool for SMT Distributors who wish to only promote their coins to real viewers.
One's settings could have an option to use the CAPCHA or not. The rewards from those who use it should be greater than those that don't do as to encourage users to comply to steem's white paper.
You are proving my point.
P.S. An even better example is Trending.