Why do they say never to melt butter in the microwave?
I think it is because butter has a tendency to melt unevenly in the microwave. The parts that heat up faster will splatter and cause a big mess or worse yet, scald you when you try to remove it form the microwave.
If I need to melt butter in the microwave, I do it on very low power and check it every 15-30 seconds and stir. I think monkemom is right. I learned this the hard way.
Be sure to cover the dish/bowl with wax paper before microwaving - no spills, spatters, or scalding. And I agree with Barbara, use very low power and keep your eye on it.
If you're careful and use low settings, you'll be fine. If not, you may burn the whey in the butter. Because of how hot microwaves can heat (and how quickly they can get there), if you weren't careful, you could have some unmelted and some burned all at the same time. I also have been known to melt butter in the microwave and do so using the "defrost" setting rather than a cooking setting.
...and be careful about "softening" yogurt in the microwave.