Of what use would be a message that would depend on american forces finding a tape in a shack in afghanistan, running it all the way back to the states and broadcasting whole on CNN. what would it say "strike now"? so if the tape had never been found no strike would occur? I don't know if you're aware of this, but usually, when communication needs to occur, the timing of the message is important. Like say, the code to pull out of vietnam (bing crosby's "white christmas") wouldn't have helped so much if it depended on the VC broadcasting it. And if you're worried about sleeper cells co-ordinating attacks, then shortwave radio could fill in that void.
I think its just useful, mystifying paranoia.
It doesn't matter to him at this point if the attacks are coordinated or not. He just wants Americans to die.
Hell he may have dozens of tapes floating around out there that will be sending signals long after he's dead.
The fact that he's planning on having his sons shoot him when the time comes and video the whole thing is pretty telling. Video of him being shot in the head would be irresistable to our media and would probably set off a wave of attacks.
I believe it could be of tremendous use to someone who first, used this means of communication, and second, was having difficulty getting his tapes widely broadcast. He may have been plotting for weeks how get a tape aired in the US, and this is his solution. After all, what better way than offering up a "confession" on a tape seemingly not meant for public consumption?