they certainly are. There is a lot of metal in a bonnet.
When I ran my fibreglass one list time I cast some pins into the leading edge (windscreen end) and drilled a couple of holes into the scuttle panel. At the trailing edge, (down by the headlights) it was supported on bonnet pins. This has a double affect in that it is then easy to remove the bonnet completely and quickly for access into the engine bay and lightness. A fibreglass bonnet is about 5 to 7kg depending on design and materials. Don't forget though in my case you can also remove the hinges which are 1.1kg, the catch and cables mechanism that holds the bonnet closed as well as the bonnet stay and other bits etc.
In my calculations it is easy to remove in excess of 10kgs. That coupled with the 20kg battery (yep!) that is currently in the car (changing to a 5.5kg) battery will remove somewhere in the region of 25kg. A 33 bonnet is just over 15kg. Things such as this as well as some other measures, I am looking to remove a good 60kg+ from an already light car. It is when you get to that level (plus a go faster engine) that it starts to make a difference. As you suggest running a light bonnet in isolation is pretty much pointless, unless you combine it with other stuff.
I would also agree that fibrglass wings make negligible difference over steel and mine will be staying. The reasons I was trying to get the carbon fibre ones off AHM is that they are a lot more flared to fit over a large wheel (16/17") and they are designed to match the 75 Veloce bumper that is sat ready to go onto the car.
So, I think, as you suggest, small bits are not really worth that much, but for me it a whole package of measures that should hopefully make the car go faster and corner better.