Depends on what you do. I've been glad to use pencil in the past, since it is water resistant compared to pen. Useful when you spill coffee on your notebook, or leave your hiking journal in the rain like me ...
I reach for a light mechanical pencil when I need to sketch a diagram or visualization since I tend to be terrible at estimating how much space will be taken up.
The article recommended Field Notes are water-proof as well, if weather conditions are a concern.
I'd think that the bigger problem when leaving a journal in the rain is trying to keep the pages from sticking together as it dries. Probably a lesser problem with the coffee unless you have a major spill.