Several reasons it is considered a loss in cycle efficiency. first, when the unit is modeled in a cycle modeling computer program, it details the loss. Second, the mixing of cold liquid water with much hotter steam implies an increase in irreversability ( 2nd law) , which implies a loss of efficiency. Finally, it is often reasosned that the steam formed by the evaporation of the spray represents steam that bypasses the HP steam turbine.
"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick