Heres what i found out by some ingame testing and calculating:
Equipping two weapons gives a 15% speed bonus
meaning average dual wield attack speed = (mainhand attack speed+offhand attack speed)/2 *1,15
in reality its a bit more complicated
While dual-wielding you attack rotation will be: mainhand,offhand,mainhand,offhand,...
So if mainhand speed after all buffs is: 1.7 , offhand 1.3
your swing timer would be first swing 1.7 ,second 1.3 ,third 1.7 ,fourth 1.3 , ...
This applys to normal swings as well as any skill you use - the skill is used with the weapon you would swing that turn
->This means that a bad offhand can give worse dps than equipping nothing in offhand as every second hit would do bad damage
->cutoff would be when the 15% speed boost gives less dps than the dps loss by every second hit being the weak offhand attack
->swapping to shield can actually increase dps if offhand weapon is bad compared to mainhand
note that looking at dps in inventory screen is not accurate, it only shows white damage, 15% haste gives also higher rage/spirit regen which means more damage trough skill use, it's a good indicator though and helped my decision to go with shield for tanking when i found a mainhand with double dps compared to offhand