Whilst the new provider cards are married to the providers box, it's possible for your sub card to have an extramarital affair with a dreambox
I don't see how providers could have a problem with this as pirating channels you don't have a subscription to will be wiped out (notwithstanding anyone that wants to risk cardsharing over the net).
If you really want to have an extra card to play with, just fork out the 67cents a day for an extra decoder.