When people post or comment and decline payout, they are sacrificing potential earnings for what they hope will be increased visibility for their post or comment.
Like people who do a lot of flagging for the sake of improving the platform, people who vote for posts or comments with 'payout declined' are also giving up their voting power and losing out on curation rewards for the sake of improving the visibility of this content.
Because there is no easily discernible self-gain, these votes have a different character from normal votes, and thus may be of interest when looking for markers of authenticity.
Just as I wasn't sure what I'd find before making the Trending Comments page, I wasn't sure what this experiment would produce, but perhaps it is useful to see how people are using the 'decline payout' option.
http://www.steemreports.com/trending-declined-payout-content/