No one has mentioned RPM and stone diameter in the Sparey discussion. Seems to me that a piece of broken grinding wheel re-cut to a half size wheel and bored (assuming an acceptable method of cutting and truing) if run on the same motor and at the same RPM as the original wheel ought to be running at that same fraction of the peripheral speed.
Likewise a full sized used wheel (if tapped and rung properly -- ie checked for cracks) might be re-purposed to a horizontal wheel at low RPM for putting a final unburned honed edge on woodworking tools, chisels, plane irons, knives, axes, etc with a fair degree of confidence. If vitrified I don't see why they couldn't be used as water stones that way, as well.
Back when I was a boatbuilder I used a wheel on a hand operated grinder for sharpening chisels and plane irons -- this grinder was geared and had a hand crank. I'm sure I didn't manage anything remotely close to 3450 RPM with them -- nor would I have wanted to, since getting a fine edge on high carbon steel requires a slow speed to avoid burning. A fine stone should be run at low speed for this kind of work, and our modern frenetic desire to shoot sparks all over the workshop every time we run something into the now standard high speed grinder isn't the only way to sharpen, nor even the best, fine as it is for removing metal in a hurry.
We seem to eschew hand work nowadays as tedious, and time consuming. But I often find it faster and easier to use my hand stones to do many small jobs than my powered grinder sitting 3 feet away.
I also find it more relaxing and satisfying than switching on that messy, dangerous, and loud machine, donning eye gear, standing away from it for a minute, and then sending sparks and abrasive into hidden corners of the workshop while breathing emery dust. Don't get me wrong -- I wouldn't be without it, but it isn't the tool of choice I go to for many jobs.
I sometimes read of people putting sandpaper onto a surface plate to "lap" some piece of work. I would suggest that they might consider re-purposing a perfectly good fast cutting open abrasive in wheel form even if they didn't feel comfortable mounting it on their grinder, and saving the surface plate for testing and marking out parts. I think you will be very surprised at the results of using old wheels as bench stones -- and they are much larger than any easily available true bench stone.
btw. If nervous about others some day using your re-purposed bench stone, just glue it to a board, or plug the hole with a glued in dowel, or better yet, a spoonfull of portland cement and sand, or mark it "Danger Cracked" (even if it isn't) or use some of another hundred or so creative ways I'm sure you can think of for allaying those fears.
