According to an article by Preston & Coleman (2000), 2 item-scale relaibility does not differ markedly from 5 item-scale reliability:
The subject of measurement was satisfaction with restaruants but it translates well to the movie rating. Ease of use, how quick it is to use and how well can a person express feelings on a differnt item-scales was measured as well. The results are as follows:
It is clear that users find 2 item-scale slightly easier to use and quicker to use in comparison to 5 item-scale but also very inadequate in expressing user's true beliefs. This indicates that 2 item-scale does not capture underlying variability very well and results in loss of variability. Discrimination indices are also markedly poorer for 2 item-scales in comparison to 5 item-scales.
Taking all of the above into account I would speculate that Netflix is willing to exchange some voting precision to lure more users into voting. I think they prefer more people voting since it increases sample coverage. This can lead to better understanding of less engaged users. Marginal value of additional information for less engaged users is likely much higher in comparison to engaged users.