1
They amplify the lineup they're in.
Put Kenny Lofton on the Manny/Belle Indians and they are completely unstoppable. Same with Rose and the Reds, or Rickey and the Yankees or Bash Brothers.
Put Ichiro at the top of a fearsome lineup and all of a sudden that lineup goes from "excellent" to "literally irresistible." It's in the consistent 3 hours of tactical pressure that the pitcher inevitably makes mistakes.
But a HOF leadoff hitter doesn't lift a bad lineup much, no.
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Can you imagine Ichiro at the top of the Yankees' lineup?!