I like the ability to leave the flag stick in...

In general I like the ability to leave the flag stick in and once you have established a protocol for your group that day, I do believe it speeds up play.  For example, I let my group know I like to leave it in from about 20 feet and further and pull it from around 20 feet and in and I’ll ask my play partners their preference.
I believe it is helpful for longer lag putting to give you a visual reference for distance,  also it could be helpful on downhill putts.  However I don’t think it is a real advantage as I have noticed plenty of shots bounce off the flag that may have gone in the hole.