From 20 years of treating cats in the clinic I can say that a large number of injuries/illness & disease in cats are caused from roaming: cat bite abcesses, dog bite wounds, hit by cars, poisonings, pellet guns, infectious diseases, fractures.... Sure, housebound cats have a greater tendancy to cystitis (urinary problems) if they don't get good environmental enrichment but free roaming cats are so much more likely to injure themselves, other cats and kill large numbers of birds and wildlife. It is also a very stressful situation for them to live in a neighborhood within the territory of a dozen or more other cats.
It is my personal oppinion that we are not doing these cats any favours by allowing them to roam. I know many people who have built beautiful ourdoor yards, much like aviaries so the cat can leave the house through a cat flap and enjoy the outdoors in relative safety and I think this is the best possible option if you don't live in the country.
These are just my beliefs - although I have been influenced by the hundreds of cats I have seen in pain or euthanised over the years.