seems to me that, in every motor sport I have ever seen, the leaders always have trouble with slow traffic. In many close races the winner is the driver who gets around the traffic best. How many times have you seen #2 or #3 pass the leader because he was stuck behind a lapped car and the second place driver was able to sling-shot around both? I've seen this in ALMS, BTCC, Nascar, motocross, etc, etc.
Slow cars have their own race to win. It is up to the faster traffic to get around them. As the previously mentioned guideline clearly states, the slower traffic should just be predictable.