Read the original SETI report (NASA SP-419) - from its inception, SETI didn't expect to overhear information-bearing signals. Instead, it was looking for deliberately generated 'beacons' - high-powered, spectrally dense transmissions. These should be detectable over distances of the order of several thousand light-years.

I seem to remember Autocad doing something like this. They used the dongle to decrypt a look-up table used in geometric transforms. If the LUT wasn't decrypted, subtle and cumulative errors would be introduced into the design which would only become noticeable after a week or two of use.

If used correctly, dongles are pretty damn hard to defeat. Ideally you use them to decrypt hunks of both your data and executable, then hide the significant decryption data amongst a blizzard of dummy calls to the dongle.