Someone could conceivably walk off with my bag and exit the customs area while I'm still stuck in the immigration line.

That's the last thing I would worry about. Not knowing whether one is flagged for secondary inspection until you exit the customs area, I doubt anyone would have the "guts" to take a stranger's luggauge with them not knowing what's inside.

It's domestic flights that I am concerned about. At airports such as YVR, people could walk up to the domestic bagguage claim area and just take off with a couple of lugguage.