Hood cowls can help engine cooling when properly designed. But this is not something most drivers will ever need.
In most vehicles, cold air is ingested into the radiator at the front of the vehicle. The now warmer air is then passed along the engine block (not a great design, by the way), and then exits mostly low down underneath the chassis.
This is fine in most operating conditions, but not in two specific cases:
- for vehicles with engines that produce a great amount of heat, because they are being pushed a great deal more than they would in ordinary use;
- for vehicles that are going slowly.
The first case concerns sports cars, or cars that are used in sporting events. The solution is to give the hot air inside the engine compartment somewhere to go, so here is where side vents come into use. Hood cowls, facing incoming airflow, also give the engine block access to cool air that (a) bypasses the radiator and (b) is forced inside by dynamic pressure.
Other solutions include siting the radiator in some more sensible position than just in front of the engine, for example in the rear of the vehicle (which is then not covered by roofing).
The second case -going slow- is often found in four-wheel drives, for example when going rock-crawling. The combination of slow speed (low airflow) and increased heat production (revving the engine to get over multiple obstacles) make overheating a possibility. This is where hood cowls, louvers, or in fact any opening in the hood is a definite advantage. Since hot air likes to go straight up, so let it. This explains your Cherokee drivers' comments.
It also explains why there are after-market replacement hoods for vehicles that need to go slowly in hot conditions, containing extraction louvers. As @Paulster2 correctly points out in his answer, these are located to the front of the hood, to avoid the local high air pressure at the base of the windscreen.
AEV Heat Reduction Hood, Photo credit: Quadratec, www.quadratec.com
So: two use cases, two different types of problem sets, and two different ways of solving it. However, it is safe to say that anything that helps get calories out of the engine compartment is welcome on a high-output engine.
On the other hand, a standard, non-tuned, engine has been designed with this in mind. Engineers have more surely set it up to cool correctly in a whole range of situations - those found in normal driving conditions. They will even have taken steps to ensure it does not immediately heat too far up when stuck in urban driving in hot weather conditions.