The reason why Campagnolo doesn't want you to install a washer is because it would reduce the number of threads that are engaged, causing additional stress on the engaged threads, possibly leading to failure. This is also why Campagnolo calls out a minimum length for the pedal's threaded section.

The reason why you might want to use a washer: because the interface between the pedal and the crank is a fretting joint, which can produce cracks in the crank. Fretting damage to a washer is far less problematic than it is to a crank.

But you should only use a washer when the crank will accommodate it without reducing the engaged thread count.