I don't use them but that's because it serves no purpose in my situation and if it serves no purpose then it doesn't need to be there.
It's added weight so regardless of how little weight it is, if it serves no purpose then it is 'parasitic'
It hinders easy access when servicing or doing any work and obscures any leaks/defective parts, this is particularly important as I service and maintain every aspect of my car.
More things to keep clean, again I wan't to make my life as easy for myself as possible.
It is a potential thing to go wrong, loose or missing clips for the cover often means the cover vibrates and this can be misconstrued as a fault elsewhere
I also don't like seeing more plastic than engine in the engine bay, it's just personal preference. Although it can look good it does require the entire engine bay to be OEM and immaculate with no colourful pipes/hoses. Just depends what sort of look your going for and how tolerant you are of these covers.
They are certainly becoming increasingly popular with newer cars and you find you spend more time taking covers off than doing anything remotely mechanical.
