Split out from http://forums.getpaint.net/index.php?/topic/107595-measurement-ruler-on-image-um-units .
Very true. I've always thought of Metric (SI) as being better for scientific pursuits, but Imperial as better for daily life. For instance, Fahrenheit is pretty well calibrated to a human being's survivability range (0° being an average temperature for the coldest areas humans tend to habitate, 100° being an average temperature for the warmest). Similarly, the foot is an easy measure of distance for a human to approximate (since an adult human's foot is within a reasonable approximation of one foot long), and a mile is about the average distance a human eye can effectively distinguish objects. A pound is about the weight of your hand, which makes it easy to approximate, and a second (which the SI uses as well) is a good approximation at the time it takes for a human to perform a complete action (about 0.25s to register and interpret a visual signal, another 0.25s to formulate a response, and about 0.5s for your body to complete the motion). It's not even close to perfect, for sure, but it doesn't have to be. Imperial is for life. SI is for science.