I use one. I am using a Rope Guide (which I love). I use it all the time. The reasons, which aren't all located in the poll, but some of which have been mentioned by previous posts in the thread are as follows:
1- Reduce rope wear. You know what I mean and you can't argue it. And sap? I barely worry about it.
2- I am a tree hugger and the FS minimizes damage to the tree. Even on a tree like an oak, when you get up to the ideal TIPs, the bark up there is thin enough that one descent will tear into and past the cambium. Like many of us here, I've worked for places where this could be the third or fourth time for that company to be trimming that tree. It pains me when I see the damage caused to the tree by previous line installations.
3- As Crispin Hoar has mentioned, it brings all trees to the same level. You can pull with the same force on your line and get the same response. It levels the playing field. I don't have to be worried about if it's a willow or birch or an oak or a maple.
4- I don't know if this is just an illusion, but I've only had the rope guide get stuck once. The rope part of it fell into a tight tight crotch and wouldn't fall through. The same would've happened had the plain end of my rope fallen in this same crotch in that tree. I used to get ring/ring FS's caught much more often. I find that a heavier FS get's caught less. I used one for a while that was 2 rings girth-hitched to a super thin, light weight dyneema sling made by Mammutt. I quickly found out that the weights on the end of such a light rope made it act like those things you see ninjas use in the movies where they swing it over their head and throw it at the feet of a fleeing enemy. When falling through the tree, my lightweight FS would get caught up in branches in the same way the ninja device would get caught up in peoples' feet. Since that lesson, i try to stick with heavy or stiff rope or webbing on all friction saving devices that I make and use.
4- It opens my options. I find that if I tie in to a natural crotch then do a natural recrotch and add a little more friction here or there by letting your rope rub on a branch, it gets to the point where it takes way more effort than is reasonable to more throughout the tree. Reduce the friction. Enjoy climbing.
There's more, I'm sure, but none come to mind right now.
love
nick