I hate outdoor cats.
That being said, it really depends on where you live. I hate finding paw prints/scratches on my car, poop on my porch and flower beds, smelling the spray from marking territory, knowing that they can carry rabies and other nasties, and feeling like my kids are not safe to play outside. I live in a residential neighborhood, and trap cats, whether they are strays or not, and they go to the animal shelter. I also work for a local animal control agency. So yes- I think if you are in a residential area, you are very irresponsible. (I also think dogs should be inside or in a fence, and not on a tie out 24/7 or allowed to roam free)
However, if you live in the country, and have several acres the cat will stay on, as long as it is fully vaccinated, spayed/neutered, and you are willing to accept the risk to small wildlife/birds, and the risk that your cat will be killed or seriously injured, then by all means, go ahead.
I really don't mean to be harsh, but I think the best thing is to keep them inside, and if you absoultely don't feel like you can do that, build an outdoor run or don't have them.