The USGA/R&A is Wrong…
The USGA/R&A say, on their site for this proposed rule change:
Unfortunately, that's inaccurate.
It turns out that there's a significant advantage to putting with the flagstick in the hole. It's like putting to a larger hole! Below, note how easily Mike McLoughlin is able to bang putts in (even when the flagstick is leaning slightly toward him):
In Lowest Score Wins, we recommended that golfers leave the flagstick in when playing all shots from off the green because of the advantage it affords you. The golf ball is more likely to go in or stay very close to the hole with the flagstick in than out.
Unless the flagstick is leaning toward you so much that the ball won't fit between it and the hole, the flagstick offers an advantage: it dampens the speed of the ball, allowing the ball more time to fall or killing speed so it stays close to the hole. Balls that don't go in because they bang off the flagstick almost certainly would have had too much speed to go in without the flagstick in the hole.
The USGA and R&A have seemingly missed this fact. In justifying the likely negligible pace of play improvements, they've assumed that the difficulty or ease of putting would not be affected.
Look at our Background Information page for more, and feel free to share or add your own data to the page.
PGA Tour Players have Caddies
The USGA/R&A also say:
This is a situation where the USGA and R&A are trying to do all they can to help with the pace of play, but in this case the juice (a negligible improvement in pace of play) is not worth the squeeze (fundamentally and significantly making putting easier).
The USGA/R&A act as if PGA Tour players will have their caddies take the flagstick out, but that's not what will happen: they'll quickly learn how advantageous leaving the flagstick in will be, and will instruct their caddies not to take the flagstick out.
Play could even become slower as each player jostles the flagstick to ensure that it's in squarely or at least leans in a way that will benefit them as much as possible.
Request for a Study
If the USGA is going to justify proposing this rule by saying this:
Then we'd simply request that they conduct a study to determine whether their guess is accurate. We feel that they'd quickly discover what we know to be true: that leaving the flagstick in makes holing putts, particularly from three to six feet, significantly easier.
We're collecting the data, quotes, videos, and other information on our Background Information page, and we encourage the USGA and R&A to be transparent about this. This proposed rule has the potential to fundamentally change the game from a sport in which we play into a uniform hole into one in which we bang the ball against a deflecting/dampening contraption that may behave differently depending on the characteristics (the density, the material, the thickness, etc.) of the flagstick.
And make no mistake about it… should the proposed rule go through in 2019, flagstick manufacturers are going to manufacture what they will bill as the "best" flagsticks for holing more putts.