By your logic, no one could need a cell phone, ever, because adults work in places with fixed landlines, and kids are in schools. But, you know, life gets in the way.
I don't appreciate students using their phones in class, but that doesn't mean they don't need them. It depends on how full their plates are - some students go right home after school, yes, but some students have a wide variety of after-school or weekend activities, and the utility of having some means of contact and communication while in transit is pretty clear. It's true that when they arrive at their destination, they are probably contactable, but you know, buses get flat tires, or sometimes there's traffic, or maybe the kid gets on the wrong train, or whatever. It's nice to have a backup in case of unforeseen circumstances.
A reasonable middle ground might be to ban cell phones when the student is under the immediate supervision of a teacher or other responsible adult. You clearly want kids to be able to call 911 in an emergency, after all, but if a teacher is around, that job should really fall to the them.