Bottom line: ONE screwed up sting operation in which heads should indeed roll does NOT invalidate the core reason and purpose for an entire organization to exist. You can beat that horse to death, but it won't change anything. As for the positive aspects of the ATF, do your own homework:
A unique law enforcement agency in the United States Department of Justice that protects our communities from violent criminals, criminal organizations, the illegal use and trafficking of firearms, the illegal use and storage of explosives, acts of arson and bombings, acts of terrorism, and the illegal diversion of alcohol and tobacco products. We partner with communities, industries, law enforcement, and public safety agencies to safeguard the public we serve through information sharing, training, research, and use of technology.
http://www.atf.gov/about/mission/