My cats love being outdoors. I live in a pretty quiet neighborhood and have a large yard so I don’t feel too worried about my cats roaming around; they don’t usually stray too far.
With my cats we trained them not to go to far as well as we could. When they were young we took them out on leashes and let them explore the yard, making sure to avoid problem areas like the street. We also ALWAYS feed them indoors, so the cats will eventually come back to eat. Similarly, we don’t let them out after they’ve eaten dinner in the evening.
As for fleas, it’s important to use flea and tick prevention medications on all cats, not just outdoor ones. Use FrontLine or another such product, especially in the summer, to prevent fleas from getting into your house. It will also help prevent flea- and tick-borne illnesses spread to your cats and even humans.
I tend to believe it’s in a cat’s nature to go outdoors. By giving my cats their outdoors time, they seem more active, happier, and less likely to destroy our furniture due to pent-up energy. But it’s important for the owner to monitor cats’ time outdoors as well as they can. You’re running the risk of the cat getting into fights or getting hit by a car, so make sure you’re very careful whatever decision you make.