Because they sometimes haven't got a clue what they are talking about.
Not kidding.
Below 75%, 50F ROP won't do any harm and it will result in a cooler engine (for the given power output) than peak EGT would, but you are burning maybe 10-20% extra fuel in this condition.
And, in cruise, there cannot be a CHT management reason for this, unless this type has exceptionally poor cooling. This would be especially true if it is rated at 100% power continuously (as most GA types are).
You would be better off flying LOP. It's cooler and you get better economy. The only downside is that airflow imbalance between cylinders can cause excessive vibration - this is why people get matched injectors sets from GAMI.
Operating at 50F ROP, or any ROP setting really, masks mismatched airflows and stops pilots moaning about vibration.
Last edited by IO540; 6th Jul 2008 at 12:08.