I think it's an excellent way to show the added value of a running quarterback, you just have to be careful how you smooth your data.
If Vick drops back to pass and scrambles 8 yards for a first down, count it as an 8 yard completion.
The problem is you have to throw out all those kneel-downs because they skew both the rushing and passing stats. They shoudn't include those any more than they should call a late-game steal that doesn't draw a throw in baseball a "steal" rather than catcher's indifference.
But it does help square Vick's apparent effectiveness in the W/L column with has passer rating. I like it.