By the time I have sprayed four coats on a car body, I have enough water in my moisture trap to fill a thimble. Not much, would you say? Well, four coats represents about 30 thimble, so it is not negligible. Especially when the water spits by drops onto the fresh paint!
If you never had any issues with water in the air line before, you can consider yourself lucky to live in a dry area (Arizona?). Even then, it is only a matter of time before you see the problem, someday. Especially if you run a long air line (more than 3 feet), as moisture can condense more in the line in this case. In short, if you dn't want to use a water trap, use a short length of air line (less than 3 feet).