Here is a list of E4 Pokemon in Black White, Black White2, HG, SS, Platinum, and Diamond Pearl.
Black White:
Shantal: None
Grimsley: None
Caitlin: Musharna, Reuniclus, Metagross
Marshal: Breloom, Throh,
Alder: Vanilluxe
Black White 2:
Shantal: None
Grimsley: None
Caitlin: Musharna, Reuniclus, Metagross
Marshal: None
Iris: Lapras
HeartGold/SoulSilver(After 16 Badges):
Will: Jynx, Slowbro, Gardevoir
Koga: Toxicroak, Swalot, Venomoth
Bruno: Hitmontop, Hitmonchan, Hariyama, Lucario
Karen: None
Lance: Gyarados, Garchomp, Altaria
Platinum/Diamond/Pearl:
Aaron: None
Bertha: None
Flint: None
Lucian: Mr. Mime, Bronzong, Alakazam
Cynthia: Milotic, Lucario, Garchomp
So to answer your question, I would say that Shedinja is useful with beating the E4 in some games. It is great at taking out threats such as Cynthia's Garchomp, Lance's Gyarados, and Iris's Lapras. But overall, I would say it is most useful in Pokemon HGSS because of how much it walls.