I'm a bit torn on this issue. All of my (seven) cats are indoor only, but all of my neighbors have indoor/outdoor cats. In my neighborhood I worry about this constantly as we are only three blocks from a four lane 45mph (read 60mph most of the time) thoroughfare, but then I also remind myself that uncountable numbers of cats in the world are completely homeless, starving and in need of medical care, so if people are willing to give a cat a warm place to sleep at night, healthy food, and veterinary care when needed I should probably be thankful. In other areas I worry about outdoor cats much less... more rural areas, quieter neighborhoods, etc. I suppose the thing to do would be to evaluate your own particular outdoor environment and the safety of a cat living there. If you feel that the cat is relatively safe there, and happy, it's probably fine (and definitely better than completely stray with no care). On the other hand, if you think that the cat is in a dangerous situation, you should probably try to adopt it out to someone with the room and resources to convert it to indoor-only (if you don't want to take on the task yourself)... or you could re-home it to a person with a better (safer) indoor/outdoor environment.
On a side note, two of my cats were indoor/outdoor until they lived here, and one of those two was *forever* trying to escape... until he squeezed out of a second story window with a two-inch gap in the middle of winter. When I finally found him he had this expression like "This isn't how I remembered it at all, mom!" and never tried to get out again. I think he spent that whole week on the couch watching television, in fact.