Everyone says this. Problem is, no one ever talks about how much money they make. So when Dropbox bumps up your salary, you feel slightly happier in isolation, but it doesn't improve the company's visibility in any significant way.
Contrast this with perks. When Google starting giving all their employees free food, it only cost them $100/week/employee, but everyone talked about it, and how awesome Google's perks are. The amount of positive PR and word-of-mouth that they got out of it, far dwarfed any efficiency improvements they might have gotten by simply giving their employees a slightly higher salary.