You haven't got it quite right. The individuals who are heterozygoous for sickle-cell get enough resistance to malaria to outweigh the (limited) negative effects they suffer. It's only individuals homozygous for sickle-cell that get the full version of the disease.
On the whole the mutation is beneficial (because having it in the population is positive) and the frequency is maintained by selection.
It'll just never reach fixation, because of the drawbacks.