Microwaved eggs was probably one of the first things that I learned to cook -- I don't know I'd consider it an 'omlette' necessarily -- it was just a bowl, oiled, broke an egg in it, a touch of milk or water, mixed, and then microwaved.
When you're rushing to get out the door in the morning, it's a dead simple breakfast; I was recently out of town, and the place I was getting my breakfast in the morning (a bagel joint) did basically the same thing -- put a laddle of egg into a container, microwaved, put it on the bagel, and you were out the door.
Personally, I'm of the opinion that velvetta melts at a low enough temp that if you're using the pre-sliced stuff, you'd probably be okay just cooking the egg, putting the cheese-food on it, leave it in the microwave (off) for ~30 sec, and it'll have started to melt.
(but then again, I like to know cheese is there ... not be some runny molten filling that runs out when you're eating something, so I don't want it too melted)
Microwaves specifically heat water molecules in the food. This turns them to steam, and because the air in the microwave is actually cool, the steam then condenses. There is often not proper air circulation to move the steam away from the food. Often times the outside edges of the food will not be soggy, but rather burnt, because they receive more energy and the water can totally vaporize.
For better results when reheating food, do not reheat your food on "high" or the default power. Reheat it at a lower setting for longer time. For pizza specifically, I heat it in the microwave only to room temperature and then finish it in the toaster oven or in a pan.
Best Answer
This probably refers to whole eggs mostly...
A microwave can boil water very rapidly, and a tight but fragile container like a whole egg will violently rupture if such rapid boiling happens inside it, because the overpressure inside it is already significant when the shell finally breaks.
Here is a video showing an experiment with ca. 180 eggs in a microwave oven, which after a minute or two rupture simultaneously and with enough force to blow the oven open, with debris being propelled several meters.