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There would be only two reasons to take NPC's along with you if you're soloing a character
1) You're an XP hogger. You don't want to miss out on any XP. This involves taking Imoen along with you through Irenicus' dungeon to clear the traps.
2) You're a fervent roleplayer. Your character would not sit by idly when a paladin needs you help. This argument is weaker IMHO, because the quests of the NPC's come up once you've enjoyed their company for at least a little while.

Both rely heavily on reload knowledge, a thing I'd like to avoid as much as possible.