If we treat people as if they have free will, then they will be likely to believe that they have free will, and develop an internal locus of control. And Benassi et al showed that people with an internal locus of control were less likely to develop symptoms of depression. So, if we treat people as if they have free will, then they will have better mental health! So, people will be better-off! And this is an argument in favour of assuming free will.