There is an interesting article at Curmudgeon Gamer on why free games should also include source code.
...the recent releases of Grand Theft Auto and Hidden & Dangerous have set my teeth on edge. These new giveaways include a binary executable and the game assets like levels and music, but no source. I argue that such a giveaway is not in the long term the interests of either the developer or the gaming community, while releasing the source as has happened with several other games is in their interests. The rule is: free as in beer isn't good enough to ensure the survival of a game for the future; free as in Free , however, can make a game live forever.
You can find the article here.