There's whirlwind, and then there's tornadoes from sprint/run like the wind. Which one are we talking about?
For whirlwind, offhand most definitely matters, since you alternate weapons to deal damage when you channel the ability. Their ticks depend on the weapon you are about to swing.
For the tornadoes you fart out, their damage comes from main hand, but the tick frequency depends on the last weapon swung when you use sprint.
Unfortunately that seems like a poor man's offhand, so no big spender will ever lay their eyes on it. People almost always put crit dmg at 90+, along with a good dps. Even with the good stuff like STR/VIT, the dps doesn't make it desireable.
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