The feature is still in the experiment mode. There will be no such button like a "Dislike" button.
Hence if any emotional smiley or button is helpful then "Sympathize" button is useful.
In this case, it is just like Facebook is toying with some features to make it available henceforth. If it gets successful then where will that button be placed?
Simply may be by placing it besides the famous "Like" button will do.
The idea of "Sympathize" button came from a hackathon. It will be experienced by many of Facebook users while surfing their News Feed status that some of their friends or any person or personality you are following has shared some sad status and you feel sympathy for them but cannot express it without using words. Then the idea of this new feature will work if implemented successfully for users.