I guess some people are less confident with keeping a bike on the road for the duration of the trip , and a new bike has more likelihood of making it to the other end .
On the other hand a cheap second hand bike has great benefits . For starters , not as shiny and attention grabbing , so you are not labelled a rich gringo so badly .
Also , if it is stolen , you lose a cheap old bike , and not 7000 quids worth . And obviously a carnet will be much cheaper , and that is also money to be lost if the bike is stolen .
I feel a solid second hand bike is a good choice . I am in South America using a 1984 K100RS which had 90,000 miles before I left . Valued at 400 pounds for the AA makes the carnet quite cheap .