Stacking Threat Reduction

[Gurnsey]

<edit: forgot - the other reason this would make sense is that the results are commutative so it doesn't matter what order you apply the reductions>
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I've seen a trend in MMORPG's in the way this works (and it's a good idea) - damage reduction is multiplicative while damage addition is additive.

Additive vs. Multiplicative damage/threat reduction:

4 items/skills at 30% reduction each:
Additive = 30% + 30% + 30% + 30% = 120% damage reduction, or you take -20% damage. So, you are healed? No good (for game designers, much as we might like it)!
Multiplicative = .7 * .7 * .7 * .7 = .24, or you take 24% damage. Much better!

Having multiplicative reduction means you can never get to 0 or negative damage, no matter how many items you have on. Same with threat - what the heck would negative threat mean? Mobs like you?

Additive vs. Multiplicative damage bonuses:

4 items/skills with a 25% damage bonus each:
Additive = 25% + 25% + 25% + 25% = 100% more damage, or do 200% normal damage. Easy to balance, not too out of control.
Multiplicative = 1.25 * 1.25 * 1.25* 1.25 = 244% normal damage, or you do 144% more damage.

Additive damage bonuses mean that additional bonuses don't spiral the total out of control (EVE Online had a problem with this with Heat Sinks, which increased damage of your weapons; while having one 40% heatsink on your frigate didn't mean much, equipping 8 40%-bonus heat-sinks on your battleship means you did a whopping 1475% of normal damage...do you think that anyone equipped anything else there? Incredibly hard to balance).