Tarps do a good job at protecting from the direct impact of falling rain, but are near useless at protecting from the secondary impact of pooling ground water. You have to be clever at finding the perfect spot to pitch to not get running ground water in a hard rain. It sounds easier to accomplish than it actually is.
.....yes, and really clever to make a bathtub floor out of ones' groundcloth and some sticks, rocks, etc.
They do present less than ideal privacy at a campground, if i'm touring to campgrounds i will often bring the lighthouse for privacy.