With the way the FBI has been handling Al Qaeda they might as well not exist for all Al Qaeda is concerned. You want to make terrorism a risky gambit on American soil? Announce that every intelligence agency has been given the green light to use any means necessary to extract information from terrorist-prisoners. Then follow that up by announcing that the military will be order to shoot to kill any individuals engaging in an act of terrorism and that the lives of hostages will be disregarded by the military while it is carrying out its mission. That would do two things: show Al Qaeda that we are deadly serious and tell them that there are officially no "human shields" anymore. And just for good measure, in major areas like NYC and DC, put SAM batteris on the police force's buildings' roofs which can be used by special police units to shoot down any hijacked aircraft if a F-15 cannot get to them first. Sending the FBI to fight Al Qaeda is like sending Jimmy Carter to play hardball with Castro.
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?