What is the effect of motherese on development?
Motherese may play a role in emotional development. 

Soken and Pick write: "Concurrent with the exaggerated speech of motherese, there are probably exaggerated facial displays, allowing infants to explore the particular aspects of the face... Child-centered displays may serve as opportunities for learning about affective events."

Walker-Andrews (1997) also suggests that "the presence of the face acts as a setting for attending to the affective quality of the voice."

Yet, according to a recent, extensive review on motherese (Saint-Georges et al., 2013), its role in emotion and cognition  is still an open area of research.

References


N. H. Soken and A. D. Pick, “Intermodal Perception of Happy and Angry Expressive Behaviors by Seven‐Month‐Old Infants,” Child development, vol. 63, no. 4, pp. 787–795, 1992.
A. S. Walker-Andrews, “Infants' perception of expressive behaviors: differentiation of multimodal information.,” Psychological Bulletin, vol. 121, no. 3, p. 437, 1997.
C. Saint-Georges, M. Chetouani, R. Cassel, F. Apicella, A. Mahdhaoui, F. Muratori, M. C. Laznik, and D. Cohen, “Motherese in Interaction: At the Cross-Road of Emotion and Cognition? (A Systematic Review),” PLoS ONE, vol. 8, no. 10, p. e78103, Oct. 2013.