Back in the late 80's I was told that the 5KG weight limit was based on what the airlines at that time would allow you check in as baggage. From what I was told, it allowed a typical pattern airplane ( Less fuel of course ) to be crated up and checked in at the counter thus traveling with the owner rather then being shipped and arriving days or weeks in advance. This would have eliminated the need to arrange someone to pick them up and store them until the pilot arrived.