And you know this how? Are you privy to everything the FBI knows and has done to foil Al Qaeda? How do you know how much information they have developed from the 1,000+ Muslims they are holding who were in the country illegally? You don't.
Not only that, excluding the war on terror, there are plenty of other reasons for the FBI, not the least of which is interstate criminals of the ordinary type. That's why the FBI was created in the first place. The bottom line is if you are going to have a Federal government, you are going to have Federal laws, and need some way to enforce them.
Ond more thing, your arguments are contradictory. One the one hand you say the the FBI is weak and ineffectual, but you are afraid they are going to impose some kind of dictatorship. How are they going to do that if they are such bumbling idiots?