That's exactly why I like to know the offset. If I know the width of the tire, all I have to do is divide it in half and add the offset to know how far the inside of the tire is from the mounting surface. The backspacing only tells me where the inside of the wheel is and I have to go through several more calculations to figure out where the tire would land.
Add that backspacing uses some unknown number (the thickness of the lip) so it's kind of a guess as to where the inside of the tire is. Like Blu mentioned above, 1/2" works and is close enough but it's still just an approximation. On the other hand, the tire is always centered on the rim so using the offset is easy.
And generally it is the tire that hits before the rim. In most cases anyway.
