I can see a few problems there. For one, the spindles I've seen are generally small diameter, which means not much surface area, which in turn leads to faster heat buildup, and that's never a good thing. For two, you're gonna have a hard time managing to keep the blade edge parallel to the spindle without accidentally messing up the grind. For three, spindle sanders are woodworking tools, and turn much, much slower then a metal grinder would, 452 feet per minute on an average sized spindle vs 4000 feet per minute on a belt grinder. The speed difference means its going to take a lot longer to do any work, and will also build up heat a lot more. The forth problem I can see is that most spindle sleeves I've seen are aluminium oxide paper, which is great for wood but lasts about half a second when used on metal.
The fifth issue I can see is having a hollow grind, but people seem to like those for some reason