Personally, I don't see anything wrong with letting a cat go outside. After all, they are an animal and naturally belong outside. They may get an occasional bird. I have three and sometimes I wish they would get more birds( don't flame me
I mean the nasty grackles that poop all over every car in the neighborhood). Like the other poster said they do keep the rat/mice/vermin population down. Not to mention snakes!!! And where I live...that's a GOOD thing. I've had coral snakes in my backyard and glad I have my little hunters out there to take care of them!
I have had both indoor cats and outdoor cats and my outdoor cats actually have lived as long or longer than my indoor cats. They are far less stressed and behave more like a cat should. My daughter couldn't keep her indoor cat anymore so we took him. He was awful. Tore up everything. Mean almost. Once he got acclimated and all his shots etc he went outside. He is now so much more happy and better behaved.
I think if you want an indoor cat have an indoor cat. If you want an outdoor cat have an outdoor cat. Just so long as you have a cat