Removing incomplete cases is so much easier than multiple imputation; why can't I just do that?
The shortcomings of various case-deletion strategies have been well documented (e.g. Little & Rubin, 1987). If the discarded cases form a representative and relatively small portion of the entire dataset, then case deletion may indeed be a reasonable approach. However, case deletion leads to valid inferences in general only when missing data are missing completely at random in the sense that the probabilities of response do not depend on any data values observed or missing. In other words, case deletion implicitly assumes that the discarded cases are like a random subsample. When the discarded cases differ systematically from the rest, estimates may be seriously biased. Moreover, in multivariate problems, case deletion often results in a large portion of the data being discarded and an unacceptable loss of power.
see this,
http://sites.stat.psu.edu/~jls/mifaq.html#cc