Interesting. I googled "microwave vitamins" and got mostly accounts of how the shorter cooking times and smaller amounts of water (or none) actually preserve more vitamins than cooking them in a pan of water. Nearer to steaming results.
There were some anti, but they did sound rather . . . ermm . . mystic. One person blamed the obesity epidemic on the introduction of the microwave (the "it started about the same time, therefore must be connected" argument) claiming "The microwave is the appliance of the living dead. People who use the microwave on a regular basis are walking down a path towards degenerative disease and a lifelong battle with obesity" No bias there, then!
It is, of course, much easier to heat something up in the microwave than to cook it properly, so there is a temptation to eat rubbish. But used sensibly, to cook fresh food for a minimum time, and remembering to eat enough uncooked fruit and vegetables too, it is not going to deprive you of nutrition. Eating a few ready meals when you don't have time or inclination to cook won't send you straight to the incurables ward with degenerative dieases either.
If it did I would be obese and unhealthy. I am a bit overweight, but a lot healthier than the average for my age. Take a pinch of salt with the diet nazis (but not too big a pinch, think of your blood pressure)