Around 40 is when I made the switch. I'd say because until that point holding aggro in a group wasnt as important as later levels. Also, more aggro weaps proc at later levels so your options are more diverse.
Also, in your 40's is when DW dps will start to beat out 2hand dps due to your primary dmg bonus increasing.
Hope that helps!
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Bonti Coas <Azure Guard> - 60 Dwarf Warrior
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