It all depends on how many people choose to read the promoted section. If lots do and the content then attracts normal upvotes and gets on to the front page, it's worth it for the author to promote. If no-one reads that tab, then it is a waste of money.

As with all experiments of this sort, we shall only find out if it works if about a month's time. If that section starts to look a bit empty, it will be because it's not providing a return on investment. If it is chokka, it is.