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Knight units have their own advantages, such as speed and morale.
And on many maps, Barbarians have the potential for a free upgrade to Ogres. And that's VERY significant. Often, it's enough to simply take one castle (if it's a Titan castle or a Dungeon) - and the Barbarian has almost won. Money accumulates, and sulfur or rubies weren't spent on it before - resulting in a quick upgrade to the highest-level troops, and a Dungeon in the hands of a Barbarian is a death sentence. And if he has ranged attacks, then Titans in the hands of a Barbarian are simply devastating.