Personally, I wouldn't want an aquarium without a canopy to hide the lights and misc. crap on top of the tank.
A well-done canopy turns an otherwise blah looking aquarium into a piece of furniture that helps blend it into the room, instead of being a bright spot that can be distracting. The glare that often ensues from a fixture to me, is distracting and is a definite drawback from an aquarium. A couple of computer-type bisquit fans is usually sufficient to remove any built-up heat from a canopy.
PLUS, retro-fit lighting set ups are usually MUCH cheaper than a fixture, and leave you with more options than a pre-built fixture.