I switched to the double stem mainly because of the way the Ortlieb handlebar bag mounts. There is a large plastic mount which stays permanently on the bars, and eats up alot of realestate. I had a Minoura Space Grip thing that stuck up above the bars to mount my NiteRider lite, computer, and HRM.

Using the double stem does a few things:

it gets the bag mount off the top of my bars, freeing up space
it drops the bag far enough so I can now see the front of my front tire (nice when riding in a group)
dropping the bag allows my light (when I need it) to hit the road, instead of the top of my bag
it will allow me to mount an aero bar for a long tour I have planned