Actually, that's an aspect that allows a whole 'nother level of head games. If you've got someone stealing your signals and using them in real-time, then it makes them vulnerable to exactly the same advantage that they are attempting to carry out. You just change the method of sending in the signal, and make sure your decoy calls the "opposite" defense and hope they try to exploit it.
What I found most interesting about the NFLN commercial where Hasslebeck discusses that (besides the fact that they were airing it while people were griping about the Pats stealing signals) is he’s on the offense, he was talking about radioed in signals not visual. It’s really interesting that Holmgren is so paranoid he has a radio play calling method that assumes signal theft, and remember Holmgren has spent almost his entire career in the NFC so anybody that want to turn that on the Pats needs a different dog. I think that’s pretty much proof positive that signal theft is a rampant and inherent part of the game, if radioed offensive signals are assumed unsafe then visual defensive signals (which are legal to steal through certain methods) can’t possibly be safe.