I do mostly box modeling in XSI, but when doing complex organic models, poly by poly is much easier. Nichod is right... just select your edges and press ctrl-D to duplicate that edge. Not as quick at just holding down control... but works just the same.
http://forums.polyloop.net/attachmen...5&d=1151767136
This guy here was accualy done with both. I did the head with box modeling and then at the open neck line I did poly modeling for the body arms and legs. Then finished off the hands with box modeling. As soon as you get down the hot keys, it goes rather fast and smoothly.