So I'm currently playing with a one-hander and shield; Not the best damage but it's steady enough.
My main-hand is an axe with 15% IAS (1.5 attacks/sec), nothing too fancy.
The other day something caught my eye:
A plain 15% IAS ring had a 500 dps increase over another with 114 str and 9-18 dmg.
So I got another, for a total of 1.94 attacks/sec.
My first thought was "I must be doing something wrong" since everyone else values every other stat over IAS, and yet the damage calculator is telling me this is a clear damage increase.
It does make sense in my head as well (more attacks/sec equals more damage), but these are the main 2 damage stats we're talking.
IAS stacks multiplicatively, so 15% IAS is a direct 15% increase in DPS. Whereas the damage and strenth is additive with your already high base damage and strenth, so their relative contributions are much lower.
500 DPS / 1.5 APS = 333.3 per hit
9-18 damage is an average increase of (9+18)/2 = 13.5
(333.3 + 13.5) * 1.5 APS = 520.25 DPS
500 DPS * 1.15 = 575
OR (just to show it's the same thing)
333.3 per hit * 1.5 * 1.15 = 575
333.3 per hit * 1.725 APS = 575
Don't know your strength so I can't do that calc, but it comes down to the same thing
More IAS will trigger hit effects more often. Faster Fury generation, faster Frenzy ramp up, more life per hit, more stun etc.
[editted out mistaken analysis]
I've been testing adding lots of IAS. It does not multiply !! So 2 rings with 15IAS each is just 30IAS. It has the same diminishing returns as strength.
I had assumed it would multiply because Weapons Master gives +10%IAS for polearms/spears. That is totally useless just for DPS ! With 100IAS elsewhere it's only +5% damage. Swords and daggers though the 15% damage is always multiplied.
some more testing results :
IAS on weapon is treated separately and does multiply other sources. Off weapon and skill IAS seems to added together.
DPS = damage * (1 + weapon IAS) * (1 + off weapon IAS + skill IAS)