Jeff,
I was of the impression that about 1/3 of groups played the 19th, but that was in the first year or two so it may not hold up long term. I have sometimes thought about just skipping the 17th and playing 19 instead, but that's a personal matter.
The 19th came about in a roundabout way. Originally Michael wanted us to put the first green over there, and in fact he keeps telling Mike Davis they should use it as an alternate green for the Women's Open ... but Jack and I politely pointed out that would bring the o.b. right too much into play, plus make for a very long walk to #2 tee. So, Michael started campaigning for us to build something there to use the space, and we couldn't argue that it was a great location. For a while we just thought we would make it a short game practice area, and sometimes it is used for that; but it was the superintendent Garret Bodington who took me over to where the 19th tee is now and asked if we couldn't make it a 19th hole.
I think it's one of the coolest greens on the course; Jim Urbina and George Waters shaped it on their own and I didn't change a thing.