Basic point - In some ciphering cryptosystem, the same data, encrypted twice would be better if it looked different each time.

This means that the enemy can't figure out that the same message was repeated.

Using a random IV achieves this goal, and using a static one doesn't.

Also, using a static IV means there is one more static part of the system, possibly making cryptanalysis easier. The more changing parts a system has, generally the better. It also means individual messages become related to eachother by more than one token. (The other token being the key).

People may put a fight saying "Oh, but this is harder, and it's a trivial security difference!"

You should ask them to tell that to the nazis, who also repeated message keys in their enigma cipher during WWI. (The message was encrypted with a message key, which was then encrypted with a session key, and sent out)

The reasoning was that it would make the message key less likely to be corrupted during transmission (these were the days of analogue), say during stormy weather, be a convenience (just like using a static IV) and it would hardly result in any change in the security of their system.

That one repetition (not dissimilar in concept to using a static IV) was the very thing that allowed the allies to eventually completely break the enigma cipher and helped a great deal in winning the war.