If you are going to use a roof rack, then a bra makes sense to protect the bike from bugs, dirt, etc. It's particularly valuable to protect the headset and brifters and the saddle cover will keep it dry in a fairly hard rain.
The downsides of any roof rack, bra or not, are noise, drag and the possibility of serious damage if you drive under a low hanging tree, entryway or, too commonly, into your own garage without remembering the bike is up there.
The current crop of well designed rear racks, and even better, hitch mounted racks, have made the roof rack less necessary. A good hitch rack will let you pivot the loaded rack out of the way of the trunk lid or rear hatch so it's not the obstruction the older types were.