Microwave ovens work on the principle of heating water in the form of liquid. When tableware is first made, it can often withstand repeated heatings in a microwave oven. Over time, however, as pieces are filled with beverages, washed, dried, inevitably left soaking in a sink, etc. they can absorb very small quantities of water. In a microwave water expands in volume as it turns to steam. The steam will usually escape safely, but over time microwaves can cause older ceramics to crack, especially if they have glazes with visible crackle patterns on the glass layer of the glaze. Rest assured I test all of my work and it survives the microwave test over many months. Unless you are microwaving the same dish twice a day, a microwave every now and then shouldn't cause any concern. That said, avoiding microwaves whenever possible will help your cherished tableware last longer.