This question is one that I always bring up when I use this book to teach about totalitarian governments. I believe that trying to "cure" dissidents is useful for totalitarian governments like that of the Party because it allows them to maintain what political scientists call "hegemony."
Hegemony (in political science) is when an oppressive government controls its people by getting them to believe in its ideas and its power. It does not have to kill people because it simply gets them to not even think about rebelling. This is why the Party wants to cure people.
The Party wants people to realize that its ideas are always right. It wants people to see that even if you oppose the Party at some point, you will always come around and "see the light." This means that it is pointless to rebel.
So the Party does this so that people will not even want to rebel -- they will realize that everyone ends up believing in the Party and that shows that the Party's ideas are correct. If you realize the Party is always right, you will not even want to rebel.
That's why the Party doesn't kill -- imagine the message it sends if it kills. It sends the message that it can only control people by violence. It sends the message that its ideas are not right. This will simply make more people want to rebel. If they cure people before killing them, they prevent this from happening.