I agree with you. And I think they are that good. Well, let me rephrase that... I don't know how "good" you have to be to keep your head down for a year or two, and only interact with 2-5 people in your cell, and come up with some simple way to "cause problems" in the US. None of the individual attacks needs to be a big deal, but if a few hundred of them happened all across the country at the same time, it would be a problem.
look at iraq - for a few days after those targetted strikes to try and kill Zarqawi, roadside bomb attacks and ambushes on US forces dropped way off. for whatever reason, these terrorists need some kind of central authority to pull these things off - same with Israel taking out the Hamas leadership - and when that structure is disrupted, so are the attacks.
i think one example of the approach you outline - was that shooting at the LA El Al terminal a while back. one lone guy, no evidence of a link to any structure, just went to the airport with a handgun and made his move.