Coincidentally, I browsed Bulbapedia and looked at the Slowpoke article a few days ago. That led me to a piece of trivia that mentioned if Slowbro receives a heavy blow or something to that effect, the Shellder will be knocked off and the Slowbro will revert to a Slowpoke. It was said that was the only time devolution has been referenced in the games, even though it is currently impossible to achieve in the games and anime. That "reversion" bit linked to the "Devolution" card article, and there you go...
Anyway, I think this would be an interesting feature in the games. That "Flareon w/Hydropump" hypothetical moveset mentioned earlier gave me a laugh. Regarding movesets, the feature would most likely help Pokemon with multiple evolutions, with Eevee being the epitome of this concept. However, most people evolve their Pokemon to have them reach their strongest capabilities in order to be suited for battle.
A "Devolution Spray" could help with accidents in the games, though. There may have been one or two times where I wanted to revert a Pokemon but couldn't since I had saved already. Plus, if Trainers don't know about the thing where "Pokemon who evolve via stones rarely learn any more moves upon evolving" (i.e., Raichu not being able to learn Pikachu's 'speed' techniques if evolved too quickly) and they save before learning this tidbit, then they might have to obtain a new Pokemon and start Training from scratch...
Another scenario is if you only save right before entering the Elite Four's building, you beat the Champion after a few hours, but a Pokemon happens to evolve post-battle. Since the game automatically saves after the "Champion Victory" cutscenes, you either have to accept the evolution or start the challenge over from scratch (a bit of a stretch, but still, it's a possibility).
As for rarity, I think if the Devolution Spray's rarity was equivalent to that of the Everstone, then that would suffice in the games. I mean, I hardly use the Everstone at all, but occasionally it does have its uses.
I wouldn't mind if the item didn't appear at all in the anime, but it could be an interesting plot point if a crime syndicate-affiliated professor used it for those movesets mentioned earlier (i.e., a family of Eevees, including Flareon w/Leaf Blade, Vaporeon w/Fire Blast, Jolteon w/Surf). Having Ash & co. facing off against such a bizarre Eevee family would be quite a sight to see, as well as raising a debate on the ethics of the "Devolution Spray" and using it strictly for battling purposes... :)
Of course, if the Devolution Spray eliminates all moves gained past evolution, then such an Eevee scenario as described above would be unable to come to fruition... :( But only time will tell should such an item come up in-game!
