I'm sure people that use the Ender client have used the Shift+Click feacher to make rally points and walked around with it. As anyone that have used it have noticed there is a huge pause each time the player reaches an intersection (checkpoint). This happens because the client needs to receive information from the server that the player have reached the destination before sending the player to the next checkpoint. People with bigger pings have it even worse as the information is delayed even more and the pause is even bigger.
I love this feacher but honestly the buggering pauses at each intersection / checkpoint deters me from using this really nifty feacher. Adding the checks on the server wont make any difference lag-wise. Coding it however might be a bit of a hassle. I dono if anyone else uses this feacher but if loft would ever read this and at least think about it it would really be a nice addon to the server.
If this feacher ever was added client side pathfinders would just be the next step. I think the biggest deterrence from seeing a pathfinder on the custom clients is the fact that even having a pathfinder its not so useful with all the buggering pauses on each checkpoint.