Deadly Poison drives many of our choices
When designing a proc effect, the developers typically lean toward one of two options: percentage-based rates or proc-per-minute (PPM) rates. Percentage-based proc rates give our attacks a flat percent change to activate, which means that faster weapons that land more attacks will yield more procs. PPM-based proc rates scale their proc chance up or down based on our weapon speed, which means we should see the same number of procs per minute, regardless of what weapon we use.
While Instant Poison and others were moved to use PPM mechanic many moons ago, Deadly Poison has remained untouched as a percentage-based poison. When we have five stacks of Deadly Poison on our target, each additional DP proc actually activates our other poison.
Deadly Poison was redesigned in this fashion largely due to Adrine's clever addon PoisonSwapper, which automatically swapped our weapons to exploit a weakness in Deadly Poison's design. PoisonSwapper would swap our off-hand weapon to an alternate weapon laced with Instant or Wound Poison whenever we were at five stacks of Deadly Poison, and then swap back to the Deadly Poison weapon just in time to refresh the stack. Blizzard had no option but to redesign Deadly Poison or force rogues into using an addon to maximize their DPS.
Because the redesigned Deadly Poison yields more poison procs when we're using a quick weapon, we want to use the quickest weapon possible. All of those extra Deadly Poison procs are guaranteed to give us extra Instant Poison procs once our target is at five stacks of DP, which means that our DP-laced weapon actually generates more Instant Poison procs than our main weapon does.
A tale of two poisons
While assassination rogues have plenty of talents that prop up Instant Poison, it's actually not that great for combat or subtlety rogues. In fact, Wound Poison deals more damage over the course of an encounter for these two specs than Instant Poison does, due to its high proc rate. The catch is Deadly Poison and Wound Poison don't synergize well due to WP's low individual proc damage. Because each Deadly Poison proc generates a massive Instant Poison proc when you're using IP, the two dovetail into a combo that combat and subtlety simply can't resist.
If you're crawling through a quick dungeon or engaging in PVP on your subtlety or combat rogue, there's nothing wrong with tossing on some Wound Poison onto your weapons. Unless you're getting more than 30 seconds of uninterrupted damage on your target, Wound Poison will probably yield more damage anyway. In fact, Wound Poison is actually the poison of choice for combat and subtlety ranged weapons in most situations, although that point becomes moot in Mists of Pandaria. The Instant/Deadly Poison combo requires a full stack of Deadly Poison active on our target before it's effective.
The quintessential quick dagger
Because Deadly Poison is a cornerstone for every rogue spec in PVE, we need a quick weapon to apply our poisons to our targets as soon as possible. Blizzard's developers have stopped itemizing quick melee weapons for rogues that aren't daggers, so all three specs are stuck using daggers in their off-hand.
While I miss the imagery of a rogue dual-wielding sharp swords or heavy maces, the fact is that itemizing gear for rogues and ensuring we have a good off-hand weapon available has become much easier. In fact, having quick daggers as the de facto OH weapon for all three specs makes it easier for us to be flexible with our choice of spec. There are other reasons to use a quick weapon, like Combat Potency, but Deadly Poison ensures that all three specs are equally inclined.
The Mists of Pandaria cloud our view
Ghostcrawler (WoW's lead systems designer) admitted that the poison system will be getting a revamp in the upcoming MoP expansion pack. While exact details of what's changing are scarce, I have to imagine that today's poisons, as we know them, will be gone. Deadly Poison's mechanics drive so many of the choices that we make on our rogues, and so there will be plenty of research and theorycrafting put into MoP as rogues work to find out how the poison redesign tweaks every aspect of our class.
Sneak in every Wednesday for our Molten Front ganking guide, a deep-dive into the world of playing a subtlety rogue -- and of course, all the basics in our guide to the latest rogue gear.