I don't expose my ammo to any element that could inadvertently make it's way to the primer or powder.
Including all the reasons described by RC, it is also a bad idea to shoot ammunition that may have any residual oil on it. When a cartridge is fired it momentarily seizes to the chamber wall until pressures have started dropping. This is a necessary function, in which, a seal is created that prevents gases from blowing by. When hot high pressure gases make contact with the chamber and breach face, eventual and repeated exposure will damage those areas.
And it is also very hard on the breach face when a case head is slammed hard against it by peak pressures. The normal function of any cartridge requires case seize, firearm damage will be an eventual and imminent result without it. One of the methods used by firearm manufacturers to test certain aspects of firearm integrity is termed as proofing. Proofing is performed using normal full pressure ammo that has had lube applied to the brass. And additionally, liquids don't compress, which could result in damaging the barrel, breach face/bolt, or the chamber.
But don't take any of our advice on this, you can pick up just about any reloading book or related literature and read it for yourself. You'll find that lubing ammunition, barrel, or the chamber of a firearm, is a bad practice and can seriously damage a firearm.
GS