This is the most common comment or criticism of this scenario.
I do not think it holds up becuase the same criteria or logic, when applied to 911 does not hold up. They easily could have done 911 earlier. Before 911 one could have made the same argument. If one said, "they are living here and will hijack airlines and crash in to the WTC or Pentagon." Using your same argument one would answer -- if they could they would have done it by now.
But they showed patience and diligence and waited for a corrdinated strike where they were successful in hijacking 4 planes and hitting targets with three of them.
Whether they have suitcase nukes or the ability to use them if they possess them is questionable. But to say they would have already used them, therefore they don;t have them does not hold up given their prior history. If they do have them they will wait for a coordinated strike.
Still think the argument holds up. Prior to 9/11 we weren't searching for terrorists as diligently as we are now. Assuming they would have them, the longer they wait the closer they come to being discovered. I think the whole "suitcase nuc" question is moot in any case. It's much ado about nothing.