The one good reason for a mouse is for people who use remote control products like logmein, vnc, rdp, citric etc. Normally the client you are connecting to is designed to be controlled by a mouse. I find these products very useful but a little difficult to use for long periods of time without a mouse. I.e remote support using an iPad with a Bluetooth folding keyboard, and a blue tooth mouse would be good.
For anything else I can't think of a use.