I guess they are useful to people doing challenges like Nuzlocke. They may give a upper hand if you didn't grind enough.
But I've never used them too. Long ago, in my Crystal version, I used a X Defense against Bugsy because of his annoying Scyther and its Fury Cutter. I didn't see any difference, actually. Maybe because I'd lost the battle either way, but this is the sole reason I tend to ditch these items quickly.
Played: Pokemon Black (and White). Progress: Finished both, with complete Pokedex in Black. Playing: Pokemon Conquest. Progress: At the post-game.