I think cell phones are a good security for kids. With as often as kids are out on their own, whether it's walking to the bus stop, or at school when there's a lock down, etc.... It's kinda a "cheap insurance" and a "better to be safe than sorry" deal.
Verizon even makes phones now that only have I think two buttons, MOM or DAD, or whoever you program in. it's perfect for really young ones. But, like Allen said, too, you don't really want to let your kids fall behind the times. I'd say at 13ish it's time to learn responsibility and cell phones are a popular way to teach it.
So, I say, if you can afford it, definately. There's not many logical reasons I can think of why NOT to have one for your kid.
~Jess