Graham,
I agree that it irresponsible cat ownership to let them roam. Mine have always been and will always be exclusively indoor cats, simply for the fact that it's safest for them. It would be the exact same thing as letting your dog out without a lead...just because you think he'll stay in the yard doesn't mean you can guarantee it. These precious little creatures depend on us having a brain in our heads to care for them, and we owe them that much for the privilege of their company.