Whenever it gets too hot, I simply use the highway pegs more... I have ridden a friends bike who had removed them and I didn't find it much different, so I have never removed mine.
My personal feeling is I like the old style lowers (without the vents) way better. My knees get colder and wet in rainy weather as do my boots... On my '97 Ultra I hardly ever got cold or wet on the legs and feet, but with the way they cut the fiberglass out at the sides and from the top on the '06 and up models, it only stands to reason you're not as well protected from the elements. Most likely this new design is to cool the HOT engines and the heck with the rider!
Besides all that, I think the vents look ugly unless on a black bike and they are hard to keep clean.
So much for technology.