Dial is a bad design for time, because most of the time people want to heat something up for a short time: 15, 30 or 60 seconds. On many microwaves this involves slightly turning a dial and then counting the required number of seconds in your head (because the exact angle is too small to select right away). A log scale dial would solve this problem, but then it makes adding another 30 seconds a non-trivial operation, because the angle is no longer constant.