Some of the best reasons for very large cards.
1. You are in a very dusty/dirty environment so changing cards significantly increases the risk of dust.
2. related, but more important - You are scuba diving with an underwater housing.
[Feel free to change your card underwater]
3. If I ever get that Hasselblad H3D-39 I will need at least 8GB cards.
Lastly while not a reason is what I would call the threshold point.
The threshold point is where you are likely to suffer a significant loss of money or reputation as a result of one piece of equipments failure.
Given that it is possible for a card to enter a condition where you think you are taking pictures but they are not being recorded correctly, the threshold for CF cards must be lower then the threshold for most other equipment.
My threshold is the 8GB card with the 1DsMkII because the occasions where I am likely to use more then 2GB of it are also occasions where only my money is at risk, and the risk to reputation is minimal on those occasions. And given when I am likely to go over 2GB in capacity, I am also likely to use both 8GB cards at least once if not twice each, both would have to fail and not be detectable for me not to get something I can use.
Note that if I were being paid a unusual amount of money and/or a once in a lifetime chance, my threshold might be at 2GB capacity instead.
Just my opinion,