With everyone and their mother carrying cell phones these days, I think it'd be more annoying if they all just rang. Then everyone would have to go digging for their phones to see if it their phone ringing or someone else's. Different ringtones make things way easier when in public, not to mention if you have the option to set different tones for different people, it can save the effort of routing around for your cell if it's someone you arent eager to talk to.