This is a tough topic. I'm on the fence with this. Personally, if I look at it like society as a whole or something of the like, the child wouldn't be able to really give back and improve life. S/he would only be absorbing resources for the duration of his/her life. Back in the past, these children wouldn't have been able to survive due to natural selection. They wouldn't have learned fast enough and would have fallen behind.
But that is the past, and we have come a long way from that, eh? Looking at the topic as an individual and a human, the child is still innocent and wouldn't deserve to be barred entry to life. Someone like this child could bring a family together through the hardships of raising him/her and turn out better as a result.
Now... for my answer? Its a mix. Life is still life, even if no significant gain is in the future. Some people grow up to be drug addicts and drop-out-give-uppers. They go no where in life, and only consume resources, just like the kid. In fact, those people usually cause more damage than this child could even understand. And because s/he would be more positive and be happy with a (hopefully) better family, the child would be better than the failures.
TL
R= Depends on what my spouse also decides. There's too many arguments on both sides. Obviously if my wife wanted an abortion, I wouldn't be there trying to change her mind. Its her fetus, anyway. Once the baby is born, I'll start arguing.
EDIT: Hah. Reading a lot of these posts keep changing me one way or another. These are pretty good arguments. I'm glad I wrote this down before I started reading other people's opinions.