Here's the vanda root that was only dipped in cinnamon without being cut. The root wasn't in active growth and dried off quickly after being watered, but even so, you can still see the tip starting to shrink back, and a split formed in the velamen's epithelial layer up near the top.
Finally, this is the Vuyl. whose roots were cinnamoned before the plant was potted back up. Its biggest pseudobulb had already started to show signs of shrinkage in the original pic, but that's gotten quite a bit worse. Since there was no change in its watering regimen compared to the same clone that grows and was photographed right beside it, something further was going on that was keeping moisture from getting into the plant.
After unpotting it, you can see pretty clearly what that was; many of the roots have started to die back from their tips, similar to the vandas but in a much more drastic way. Given what's going on here, it's a pretty good bet that the cinnamoned roots would eventually all just die prematurely over time.
I learned something from this whole thing, but I think it's important to remember that these "experiments" were not conducted rigorously at all, so we can't come to any hard and fast "conclusions." As Rob pointed out, cinnamon may very well kill fungus, mold, and bacteria if it's applied in direct contact with the stuff; the problem is actually getting that to happen. Commercial plant disease controls are mixed with a chemical "delivery system" that gets the active ingredient past a plant leaf's natural barriers, and ground cinnamon contains no such system.
However, it's been made pretty plain, at least to me, that using cinamon as a preventative or cure for root rot can cause more damage to healthy roots than it would seal, or "fix" any rotten ones, so I for one am not going to recommend using it any more, either for leaves (because of its ineffectiveness) or for roots (because of the real and potential damage.)
If anyone has the time, inclination, or means to carry out more extensive and thorough testing with this stuff, given the prevalence of cinnamon as a "cure-all" in popular orchid culture, I think such testing would be well worth the effort.