I just noticed this otherwise I'd have posted sooner.. The actual main point of these "free listen" campaigns isnt to win back customers (because customers who have had the service and canceled know what they had), its to give Sirius opportunities to kill unsubscribed radios that may still be receiving free programming because they missed their original deactivation signal for whatever reason. Sirius was publicly embarrassed a couple years ago because an article pointed out that due to the flaws in their Conditional Access system, something like 30% of radios that had been deactivated were still receiving programming for free.
So basically they use the "previews" every couple months as a two pronged attack.. On the front end, it knocks basically all the good/valuable content (sports, talk, and uncensored channels) off an otherwise still-working unsubscribed radio and leaves you with just the worthless crap. Then once the "preview" is over, they dump kill signals into the stream to turn them off.
There isn't enough bandwidth in the system to keep the kill signals in permanent rotation so they had to go this route.
Sent from my Galaxy SII using some Tappity Talkity thing.