i mainly do the bird/wildlife photos, i never know if i will be pointing the lens into the light, shade, a dark tree, a hole in a log, and when i do snap a shot often its a need to shoot now before its gone situation so i am not sure a card is going to be helpful for what i usually shoot. most wildlife guys i have talked to seem to use auto white balance and that is probably why.
for landscapes i might try to start using one, ill see if i can find some laying around , i guess my main concern at the moment is just getting a well composed photo when it comes to landscapes.