(Looks a lot more legit than my State of Maine ID)
I'm not entirely sure why it's necessary-- I've gone more than two months without one and haven't had any problems-- but they are apparently something that every Belgian resident over the age of 12 has to have. According to Wikipedia, everyone over the age of 15 has to have the card with them at all times unless they are within 200m of their homes, but I've never heard of this, so it either isn't true or isn't enforced. At all.
Despite everyone saying that the ID is absolutely necessary (the police even have to come to the house to make sure I'm really living where I say I am) I have only been asked for my carte d'identité under two circumstances, which were getting a bus pass (my passport worked fine) and enrolling at the library (there was nothing they could do for me without my card). I've heard stories of people being asked for their identity cards when buying alcohol, but this seems to be something that happens only rarely and is easily bypassed by saying that you forgot your card at home.
At the very least, now I have a colorful Belgian souvenir... and it's good until October 2012, so if I want to come back right away, I still have that option.
À la prochaine!