I think the vents were added/designed to give rider more air to legs then to reduce heat on the engine. I personally like the look of my lowers on my bike and have never taken them off. I also believe the lowers were designed to capture/force more air to the engine. Granted it's only forcing it to the front jug, but by design of our engine with or with out the lowers isn't going to direct anymore air to the rear jug. I try to avoid situations were my bike is not going to be moving (getting airflow) when it gets really hot. If I'm going to be in situations where it's that hot and standstill traffic I want to be in my cage with the A/C on Max. 

Ride Safe,
Fired00d

True...it probably helps a little more air get to the engine to have the vented lowers, but I don't see the lowers blocking any more air than your legs do, and I prefer the looks with them on. I was in 100 degree heat last summer on numerous occasions and bike never went into overheat mode.
After all, it's the rear cylinder we all worry about, and having the lowers off ain't going to help that situation. I have noticed that the Dragon Wings (or the other adjustable "wings") do help a WHOLE lot in directing more air to your upper and lower body while moving, so I personally think it's helping cool the rear jug off a bit as well to have the wings open and directing the air where you want it. JMO