My DD has a pre paid mobile, she pays for her own credit with her pocket money so she knows to be careful with it. I like her to have it incase i need to get intouch with her (if i have to rush out and wont be home when she gets home) or if she missed the bus etc. Sure she wastes alot of money and time texting but its her money and as long as her homework is done its fine, plus then shes not running up my phone bill! She takes it to school but leaves it in her locker during class, sometimes i get called at lunch time to be asked if she can walk home or just sometimes to say shes having a bad day.
I guess for me i like knowing i can contact her and vise versa whenever needed.