I'm on the fence about this. There is no vaccine and I don't know enough about it to know why on can't be developed.
However, I don't agree with Val about there being a reason for it in nature - smallpox was a "natural" disease that killed millions of people, and everyone is glad it is no longer a threat.
AIDS is a disease that started in "nature" yet it too kills millions of people, and we'd love to see that eradicated. Cancer, too, is a naturally occuring disease, and we would do almost anything to find a cure - isn't that what folks have prayed for for years and years?
I see no valid purpose for keeping malaria alive - there is no benefit other than to the parasite which causes it.
However, I'm not too keen on genetically engineered insects, period. I would much rather see efforts and dollars spent on a preventative vaccine that can stop Malaria from being caught, rather than releasing millions of genetically engineered insects out into the world.