Oh. So you did understand, after all :-)
How did I know that, I wonder, eh ? :-) OK.
Cells are not self-sufficient. They're not the source of terrorism, only a tool thereof. Terrorism is fostered by states and other players with suitable resources, who feel inadequate -- not strong enough -- in the conventional military and political realms; for them, under the cover of plausible deniability, terrorism may provide an effective alternative. It is these sources that need to be nipped in the bud.
Saddam's Iraq was one of such sources: Saddam was training Al Qaedistas in Sudan; Saddam's security men are likely to have been in contact with Osama boys in Prague; Saddam -- under sanctions, when hordes of western phoney baloney left were howling 24/7 about the "Iraqi babies" who were purportedly dying by the million because of Saddam's (also purported) lack of funds -- appeared to suffer no such shortage of funds when it came to shelling out $30,000 per pop to suicide bombers massacring civilians in Israel ($30K goes a long way in the Middle East, in fact, it's a good annual pay even in the U.S. -- a lot of good working americans make less than that, while blowing up no one; I suspect these hundreds of thousands of dollars could have helped to save a baby or two; incidentally, I don't remember the western "humanitarians" raising the issue.) Saddam hid the mullah fucker who blew up the WTC in NYC the first time; Saddam was covertly getting dangerous stuff from North Koread and the Serbs; Saddam gave refuge to Abu Nidal (why would he do that? out of goodness of the heart?)
The UN terms set in '91 expressly forbade Iraq to have missiles flying farther than 150 km: Saddam proceeded to developing rocketry with the reach of 1000 km. Saddam failed to account for 1.5 tons of VX, 1,000 tons of mustard gas, and 550 munitions containing mustard gas. Saddam has neither signed nor ratified the Chemical Weapons Convention. After '98, when UN inspectors departed, there was no verifiable information about Iraq's biological weapons program. Saddam reported of some armaments as "destroyed" though according the the UN rules he had to hand them over, thus, they had effectively remained unaccounted for.
Needless to say, Saddam was doing all this while ignoring the UN resolutions and sabotaging the weapons inspections since '91 (over a decade, that is, after he was kicked in the butt the first time over his invasion of Kuwait in '90.) Saddam was getting more and more obnoxious coz he believed (correctly) that time was working for him -- his friends the Russians and the French (to say nothing of "the Arab World") were clamouring more and more loudly about even ending the sanctions (already sabotaged and compromised; viz contraband oil trade controlled by Saddam's sons, and don't forget to check out the Oil-For-Food scandal brewing right now in the UN.)
There was probably more covert activities there -- given the kind of totalitarian control he exercised over the country and the UN inspectors having been kicked out in '98, we couldn't possibly hope to know of everything he may have been doing. Neither did we need to -- the above was more than sufficient to realize very clearly that, in addition to a brazen affront to the UN, his general political stance and obvious intentions represented a grave security threat -- especially after the 9/11 attack.
The other "rogue" players were looking at what was going on, and if Saddam had been let off scot free after all this, they would have got some very wrong ideas: among other things, Saddam needed to be dealt with simply pour encourager les autres, so to speak.
Now that particular source of terrorist support (along with other security threats Saddam's Iraq represented) is gone. In the bargain, the suitably encouraged Qaddafi seems to have got the lesson (not a bad side effect.)
To drain an inundation it won't do to bail water with a bucket (though you'll probably need to do that too.) You need to shut the fucking floodgates. Therefore, taking out cells is a necessary but not the core task: you can fight them till you're blue in the face and they'll keep reappearing so long as there's organization, finance, training, political and ideological support, etc., etc. Yet cells will disappear if starved of such resources and support -- not at once of course, but meantime you'll be able to fight them much more efficiently.
That is a larger context of and the main goal in the war on terrorism. That's what they are at the moment doing in Iraq. Whether you like Dubya or not, what they do just works, they do the right thing.