Why bother doing if it doesn't do any good? Golf teaching guru Dave Pelz in his book The Putting Bible says it doesn't work. Good enough for me.

Plumbing the putter is used a tool to see the level of the green and give you an idea of the slope of the green and the direction of the putt.  I use it when I am in doubt of the level.  It does not however give you any idea of how much break to play.  But once you get an idea of the level you can then look at both sides of the hole and stand on the "low" side of the hole and see the green easie for a better read of the putt.