Hmm, that's odd. I used to use Randowhale, Bellyrub and Buildawhale relatively regularly but never seemed to get back as much as I put in. But then again, I never actually did the math, it just felt that way. All of those bots had pretty decent Steem Power when I used them.
But yes, I definitely agree that it's mainly for the exposure than other reasons. Anyway, that was just my personal experience, and like I said, I never actually did any math on it :)
RE: Random selection of vote purchases