A day in the life of ..

[Klaus]

Shadow weaving is refreshed every time you use a power on it -- add 3s from a mind flay, and maybe another mind blast, and you come out to having shadow weaving last the whole time that shadow word: pain, does. The important thing is to make sure SWP outlasts shadow weaving, to ensure you get the maximum out of it -- plus, hey, another .. 300ish points of damage in my current +dmg gear for a talent point. I've thought about dropping mental agility and maxxing swp, shadow reach and mind blast, and still spacing them out with two mind flays -- simply because as shadow, there are only a few instant spells that benefit from this ..
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But weaving stacks 5 times - every time you add a flay or a blast, you'll get another 3% from weaving, and I was under the impression that each one lasts 15 second (i.e., they unstack sequentially instead of all at once). So as long as you're hitting with shadow spells, some amount of weaving should be active at all times. So, either your SW:P runs out or the target dies before weaving fully expires (unless you quit hitting it with spells, of course). So I guess I was unclear on why you would put 1 point in SW:P and not 2. The only reason I could see was if you expected the target to die in less than 18 seconds, in which case the extra two ticks never come up (and the talent points are therefore wasted).

I might be leaning towards agreeing with you on the Mental Agility. Instant cast spells that come to mind are: SW:P, Devouring Plague (I'm undead), Shield, Fear, PW:F, Inner Fire, Shadowform, Fade, Renew, Dispell, Cure Disease, Vampiric Embrace, Silence. Some are obviously not usable in shadowform, and others (VE) have such a low mana cost to begin with that Agilty doesn't make any difference. Of the ones I would use in shadowform (SW:P, Plague, Shield, Fear), the savings would be about 100-200 mana per fight (SW:P being the only one in that bunch that I use on a regular, every-fight basis, and savings there is only 47 mana). However, the reduced cost for Renew and Dispell would be very nice in some fights, especially with the increased cost of dispell/cure in the latest patch. I remember some fights where I feel like I'm casting dispell constantly.

Another option would be to spend 3 points to get focused casting, leaving you with 2 points more in shadow - one for SW:P and another for shadow focus? Focused casting would be handy for solo work, but I'm not sure how much it would help in group situations.