And people really need to stop running Shell Bell on things not using FEAR.
I think Shell Bell could theoretically be useful on something with a great offensive stat (Attack or Special Attack) and decent defenses, but a crappy HP stat, so that the health regained from the Shell Bell would actually be better than that from Leftovers. The problem is that there's no Pokémon which fits this profile.
In practice, I've ground that Shell Bell is like a neutral nature - only useful in-game as a sort of poor man's substitute for the optimal but rarer alternative. In my
Pokémon X game, I run a Docile Blaziken, but only because I don't have an Adamant one (there's five neutral natures and only one of each particular plus and minus nature, so a Docile Blaziken is much better than, say, a Bold, Modest, or Quiet one and much more likely than Adamant). As soon as I have a good reason to bench Blaziken for a while and don't have the Daycare occupied (trying to get a female Squirtle at the moment so I can breed Aura Sphere onto it), I am sticking Blaziken into the Daycare and churning out Torchic eggs until I can get an Adamant one. Until then, Docile's good enough.
Shell Bell is the same sort of thing. The only reason you'd use it is because you don't have enough Leftovers to give to every 'mon you want holding one. (And even then, Sitrus Berry is a better "poor man's substitute" unless you
really want your 'mon to have constant recovery.) On a simulator, you can actually
choose your 'mon's nature and held item freely, with none of that pesky "logistics" or "scarcity" stuff to get in your way. So there's really no excuse for a suboptimal item or nature.
http://replay.pokemonshowdown.com/ubers-78523967
Only two actual Ubers in this battle... and then it only got worse.
Turn 1
Victini used Bolt Strike!
It doesn't affect Gliscor...
Me: "Do you have
ANY idea how to play this game?!?"
Physical Alakazam and Mewtwo using Psychic and Thunder (in sunlight) made me just stare slack-jawed at my screen, struck dumb at the sheer
incompetence of the Trainer. There is just no excuse for that sort of insanity. And the Azumarill was equally dumb. (Scald and Ice Beam?!?
Really?)