The military requires sealed ammo because they will have guns that are exposed to both fresh and saltwater, in some cases for extended periods of time.  Imagine a modern equivalent of the D-Day invasion, having to jump over the side of a landing craft into 20 feet of water 150 yards from shore.

Ammo in a sealed ammo can is protected by the can, sealed or not.  A little rain won't hurt your ammo either, nor will crossing a stream and dunking the mag for a few seconds.

On the other hand, high humidity for hours and days CAN damage your ammo if it isn't sealed.  Water vapor in the air will naturally want to equalize with the dry air contained in the ammo cartridge, and will do so over time.

A good example of this is in a car, especially the trunk.  Many folks here can tell you a story of how they put a gun in their trunk, and, months later, found the gun to be a rusted mess.  That happened because any moisture in the trunk will eventually vaporize and cover everything, plus the large temperature swings a car experiences leads to condensation forming.  The same problem will affect ammo.

If you live in an area with high humidity, it should be a concern for any ammo not stored indoors or in a sealed container.

-Troy