Nectar robbing commonly has negative consequences for the reproductive success of plants.
Because total plant fitness is the result of both female and male functions, Rojas-Nossa et al. studied the effect of nectar robbing on several variables used to characterize both sexual functions of Lonicera etrusca, a pollinator-dependent plant with long tubular flowers that produce abundant nectar. Although nectar robbing was very frequent, no negative consequence for plant reproductive success was found. They propose a combination of morphological and ecological mechanisms that allow plants to compensate for the energetic loss caused by robbers.
Sandra V. Rojas-Nossa, José María Sánchez, Luis Navarro, 2015, 'Effects of nectar robbing on male and female reproductive success of a pollinator-dependent plant', Annals of Botany, p. mcv165 http://dx.doi.org/10.1093/aob/mcv165