I don't think it's fair to say he draws zilch or piggy-backs off of other fighters.
Let's not forget, only a year ago Rousey/McMann only did 340k buys. She has only recently become the top PPV draw, she wasn't big until 184.
175 was mostly Weidman carrying the card. This was before Rousey's 16 and 14 second KOs, movies and WWE appearance that bolted her into superstardom.
Also, 187 should be considered an outlier. Everyone predicted it's buys would do poorly because of the Mayweather/PacMan hangover (it was less than a month later). The fight was hardly promoted for this reason. Not to mention the whole jones fiasco hurt the buys.
As of now, Weidman is a middle of the pack, 400-600k buy champ. However, I believe he can be a consistent solid draw (600-700k). The reason he is on this card is simply because he said the earliest he was looking to fight was December (because of his wife's pregnancy) and it just played out that way.