I dunno about you but while Deadly poison does do decent damage, it takes times to run its course. And when you're a rogue and not a warlock, sometimes you can't wait out DoTs. Sure it'll do more damage in the long run but in that long run you're still getting beat on and Evasion, etc. will only hold out so long. I would rather use Envenom since without any of the other poison talents I have little trouble getting five doses of Deadly on someone.
And if you use Instant, what's the point? You may as well 20/41/0 since Mutilate requires daggers like backstab and ambush. I dunno I just think if you want to expound on poison application and not using envenom you may as well just go all-out and go Combat swords, maces, or fists.
When talking about a mutilate build, I have some basic concepts that drives how I play, given a particular circumstance.
My first concept is that poisons, in and themselves, are simply there for mutilate to generate damage from. Even a simple crippling poison will allow mutilate's damage bonus to process.
When fighting a single trash monster, I am not concerned with DoT. I am also not concerned with finishing moves, whether it's eviserate or envenom.
When fighting a boss, I am more concerned with DoT and sustained damage, not neccessarily burst.
In concept #1 and 2, when dealing with single mobs, my stun cycle will kill them off faster than waiting around to do a finisher. In that regard, if I'm farming, won't finish with evisterate, I'll hit SnD, so that on the NEXT pull, I'll have remoreless AND SnD up. Coupled with my haste proc from my D3 set items, I'm simply a killing machine. Once I get rolling, I can wipe out an entire area, simply by CS/Mutilate/KS/Mutilate/Mutilate/SnD (dead) cloak wash rinse repeat.
When fighting a boss, everything changes, as now I'm more concerned with DoT and sustained. Mutilate is more for gaining CP's fast to maintain my cycle of SnD/Rupture. When attacking a boss, it's garrote/CB/mutilate/SnD/mutilate/rupture/mutilate/SnD, etc. The only thing that holds me back is fear of gaining aggro, because of the amount of damage I'm outputting. I also use anesthetic poison to mitigate my damage output, and to add yet another DoT to the boss. Once the Deadly poison has run it's course, I'll toss in an envenom and start up the deadly DoT again.
Finally, in PVP, I use crippling and wound poison, thus envenom isn't of any use to me.
I personally feel that anesthetic poison is very much overlooked and ignored by the rogue community. While some might argue that because it's ignored it should tell me that it's for good reasons, I tend to dismiss comments as mob mentality. I've had other rogues tell me that if I'm not taking quick recovery i'm worthless too. To me, if a healer is having to heal me, something else is wrong. Shrug. Blizzard only allows us just so many talent points to distribute, and if I want to do a 41/20 build, I need to maximize those 41 points in assasination to do as much damage as I can. I have to sacrifice what some rogues would call "essential" skills to do so. If I listened to every rogue who tells me I should have such and such skill, my talent tree would need a total of about 70 points.