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Both "forced EXP share" and "no EXP share" still ultimately encourage you to use a single core Pokemon as your powerhouse because Pokemon is a game where 99% of its campaign battles are one-on-ones.
No EXP Share means you'll want to focus on a single Pokemon to beat the level curve without having to grind up new party members, since they're better off being sacrificial fodder to revive your powerhouse.
Forced Party-wide EXP Share means you'll want to focus on a single Pokemon to beat the level curve while new party members passively build their way up to be within the proximity of the level curve, which subsequently allows them to have further versatility beyond being fodder to revive your powerhouse.
Lastly, though, none of it fucking matters for Raid Battle Era games. Literally spend an hour throwing your best Pokemon at laughably easy low-tier raids and then cram all of that unfathomably rewarding EXP Candy down your best Pokemon's throat and watch as they completely eclipse the campaign's level curve.
