It takes away from training time so it slows his development. This ultimately leads to players not reaching their max pa. Also adaptabiltiy is a hidden attribute (download genie scout if you want to see it). The higher that attribute, the faster they will be able to learn new positions. I rarely use this unless I am managing a lower league side that can not afford to sign many players and I need to make players that can run out in a variety of roles.
Hope that helps!