I thought the purpose of these was so that as tips are changed over the life of the shaft, the fiber pad is cut instead of the ferrule. Now, a repairman who does tips well shouldn't cut into a ferrule much. About a hair at most. But with this, even that tiny bit that is cut each time is the pad instead, rather than the ferrule. If you get 3-4 tips a year, and keep a shaft for 15 years...the ferrule can end up noticeably shorter. Altering how it plays, and possibly harming the value. The fiber pad gets cut down too of course and doesn't last forever, but it can save shaving down the ferrule for about half a dozen tip replacements. Also, the ferrule gets cut once to be squared perfectly and that's that until the pad itself needs replacing. If you do your own tips without a lathe, when cutting off a tip and sanding/scraping the top to reface it, if you accidentally make it concave or convex..not a big deal, do it again and get it square, or take it to a cue repairman to have it done on a lathe. That way, the ferrule isn't damaged or taken down too much due to an error.
Also beware of overzealous cue repairmen that don't quite understand what a couple thousandths means. The only amount that should be cut is enough to remove the old glue and make the surface nice and flat. With a fiber pad on there, you can instruct a repairman that you're unfamiliar with because you're using him for the first time - not to cut it off, and instead cut into the fiber pad. This may also save your ferrule from being chopped down. Also, if the guy doesn't know what you're talking about, is giving you a hard time, or doesn't "get it" ...might not want to have them do the work.
If all that means something to you, go for it. If you don't change tips often and don't keep a shaft for so long, it doesn't matter. On cheap production shafts that you might go through every few years, it doesn't have much of a benefit. However, if you have a custom cue, with a shaft made by someone who isn't alive anymore, or is too expensive to replace, and you never sand this shaft and want to keep it a lifetime - AND have it play the same for that lifetime, seriously consider using these fiber pads to save the ferrule as much as possible, and also consider finding a competent cue repairman or cue maker to do tip jobs. Or learn to do it yourself and do it right.
Yes, ferrules can be replaced...but on some cues the shaft doesn't play the same as the original ferrule.