First of all, remember that every player that you are seeing when you watch the pros has spent his entire life putting with the flag out on the vast majority of the thousands and thousands of putts they've hit. They are used to it, and change comes slowly in golf and especially on Tour. Secondly, remember that their speed control is nearly flawless, and there just isn't much way the stick is going to help them. And I believe that I did see players at Pebble leaving the stick in on long putts that in the past would have been tended; Woodland's chip on the green on Sunday is an interesting example.
As to the studies done so far, all test the same thing, I think, which is the effect of the stick on a rolling ball. What I don't think you can test is the effect of the VISUAL element of a smaller, more specific target on effect putting by non-Tour players. When you teach free throw shooting to basketball players, you teach them to focus on a VERY specific point on the rim, rather than the general area of the basket; most say the back of the rim. Smaller target, smaller misses. It's the same thing with full swings in golf, using an intermediate point or a tree in the background, etc.; alignment and focus get better as the target gets smaller. I see no reason to believe that putting will prove to be any different.
A personal story: My regular group and I fell into the habit of leaving the pin in while putting from the beginning of the year, and I've continued that into tournaments. Yesterday I played a local senior tournament at a (very) public course, and all but a couple of the flagsticks were badly tilted; I wasn't sure if it was because the holes were too deep, or the bottom of the flag was slightly the wrong size, but all of them were leaning, so I putted with them out in most cases.
And I putted horribly on shorter putts with the pin out. I overplayed the break all day; consistently ran the ball over the high edge of the hole. What is usually a strength of mine in competitive rounds quite literally cost me the tournament. That's a VERY small sample size, but really made me scratch my head about the visual element of all of this.
So maybe forget about whether or not the pin helps or hurts a rolling ball; I suspect studies will show both to be true, depending on how the study is set up. And assume that pace of play in casual rounds gets better. That leaves the visual element as the wild card, and while I don't know how that can be studied, I have a feeling that it's a big deal.