Great question, Walt.
I walked down the hall and asked Bruce Sanderson if he swallows when evaluating wine. "Only if the wine is good," he answered with a smile.
When you're evaluating flights of wines, you need to keep your palate--and your brain--clear. Spitting is key; you still get the finish with its (hopefully) lingering flavors and (hopefully without) burning alcohol.
Before you spit or swallow, swishing the wine around in your mouth releases all the the flavors that resolve on the finish. At the same time, any excess alcohol is certainly making its presence known through your sense of touch.
Bruce suggested that you try spitting as you evaluate a flight. Then go through the flight again, this time swallowing as you evaluate the wines (make sure it's a small flight!). If you really pay close attention when spitting, you probably won't get more info when you swallow.