I have just moved mine down from the roof, onto a bonnet with the depression, however I can already see the disadvantages, in that it does restrict forward vision a little (and these are not fat tires) and it does require you to be a weight lifter to open the bonnet.
However I guess it will stop the bonnet trying to blow off and I can still see the wing mirrors unrestricted, so I don't excpect the loss of forward vision is going to be that noticable out on the road.
I reckon it is more sensible for weight distribution than up on the roof an easier to get at anyway.