GTAW keyholing to me is an unnecessary complication for most alloys. The only time I would recommend it is sometimes with certain high viscosity materials like the Monels (especially if the bevel can't be layed back), or even some Nickel/Chromes, or very low sulpher SS's (these are notoriously inconsistent in penetration), or on repair procedures as such. But you are right, when you keyhole it you can watch the edges consume (though it does work better with smaller torch angles requireing that you lean forward a little more to see-OK on bottoms, a little tougher on top), you don't have to look for subtle puddle characterisitcs to let you know you are penetrating.