So when I bought my Sky+ HD box some 10 years ago, I dont recall able to see "the main" free channels without a viewing card. I recall purchasing a card from Sky for about £20/£30 upon which I could then access the free channels. Whenever I removed this card from the receiver, the channels were scrambled again. I was told that the card would last about 5 years upon which I would need to buy a new one.
After some 5-7 years, I recall taking the card out and the box would still display the main free channels. I have never had to buy a replacement card.