Well i think just because its not a "full fia" cage shouldnt mean its useless...its for a looks point of veiw, would look more silly having 2 bucket seats with no cage in my eyes, and its still a road car and i dont want to ruin it by putting a full cage in, its just for looks and to hold my seat harnes like the pic above, and plus i like the look of them, when your behind a car and you see some pipework you know somone has got motorsport in mind 
....I wasn't referring to a "full FIA" spec cage. So, are you after a properly mounted half-cage? If properly mounted it will serve to protect in an accident. However, there is a school of thought which says that if you have a roll-cage fitted and are not wearing a helmet (which you wouldn't do on the road) then you could more easily suffer head injuries.
When you wrote "just for the looks" I thought you meant a 'Show cage' - Many of which aren't even mounted properly.
I'm afraid that fitting any roll-cage will involve cutting into your interior trim etc and must be securely mounted properly if it's to use a safety harness.
Insuring a car for road use with a roll-cage can be difficult too, but SteveP (or Obese? Or NeilM) should be able to advise you on that.
