With wireless devices, it's all about convenience and portability. If I have to pack up my laptop for a business meeting, there are less wires, and you can place your mouse and keyboard where it's convenient. I tend to use my left hand for mouse even though I'm right-handed, but sometimes I switch...easy as pie to move it around. With keyboard, if you have a keyboard drawer, I always remember fumbling with the cord under the desk. Again, very convenient if wireless.
Main con for me is that the wireless mouse or keyboard takes up a USB connection, so make sure you have extra USB ports. And batteries too for the mouse. I tend to forget to turn my mouse off (switch at bottom).
Hope that helps.