I've been leaving it during solo rounds for years. Glancing blows to the flag can knock the ball off the cup, but it's rare. Power lipouts, ball can be slowed down by the flag and stay on/in the cup, but it's rare.
And the speedy putt can hit the flag and go in too. Other than that I don't see much impact to leaving the flag in vs. out. It certainly is quicker, but will it matter on a jammed up course. Everyone would have to do it to experience that benefit. Traditionalists I don't think will. as for pace of play as whole, we are so far gone on that issue, that things like flag in, won't make much of a difference.