First off SM/Ruin is a very viable MC/BWL spec, especially for alliance with Blessing of Salvation, the Demonology tree has some great utility though.
That said there are two popular raid builds involving demonology. The first is the very common MD/Ruin. Core of this build is raiding with the imp for the extra threat reduction, since that's the real limiting factor to your damage (especially in BWL on bosses like the Broodlord.
The second build is SM/DS. The idea here is to sacrifice either a voidwalker for great health regen (great for bosses with AOE, especially when combined with siphon life) or the succubus for a whopping +25% shadow damage to all your dots and shadowbolts. You'll miss out on the faster shadowbolts though, which can be a pain on certain bosses like Chomaggus where you want to get in and out of line of sight as fast as possible.
Neither of these builds is great for PvP, with MD/Ruin you'll be forced to seducenuke, the second will rely heavilly on DOTs.
I've provided some sample talent builds, most of it is pretty straightforward though the points in Improved Voidwalker/Succubus could be shifted to Fel Intellect/Stamina and Improved Shadowbolt could be replaced with Cataclysm if you have a low mana pool.
Playing these builds is not much diffrent from playing a SM/Ruin spec, DOT on things that last at least the full duration of the DOT, CoD on offtanked adds and such, then spam away on the shadowbolts.
As for why not to take soullink: you'll mostly run MC/BWL with the imp out, and he doesn't have enough health to soullink with safely, and neither does the succubus; especially in AOE situations.
Hope that helps, and best of 'lock. :P
Edit: typo.
Edit2: Sample builds.
That said there are two popular raid builds involving demonology. The first is the very common MD/Ruin. Core of this build is raiding with the imp for the extra threat reduction, since that's the real limiting factor to your damage (especially in BWL on bosses like the Broodlord.
The second build is SM/DS. The idea here is to sacrifice either a voidwalker for great health regen (great for bosses with AOE, especially when combined with siphon life) or the succubus for a whopping +25% shadow damage to all your dots and shadowbolts. You'll miss out on the faster shadowbolts though, which can be a pain on certain bosses like Chomaggus where you want to get in and out of line of sight as fast as possible.
Neither of these builds is great for PvP, with MD/Ruin you'll be forced to seducenuke, the second will rely heavilly on DOTs.
I've provided some sample talent builds, most of it is pretty straightforward though the points in Improved Voidwalker/Succubus could be shifted to Fel Intellect/Stamina and Improved Shadowbolt could be replaced with Cataclysm if you have a low mana pool.
Playing these builds is not much diffrent from playing a SM/Ruin spec, DOT on things that last at least the full duration of the DOT, CoD on offtanked adds and such, then spam away on the shadowbolts.
As for why not to take soullink: you'll mostly run MC/BWL with the imp out, and he doesn't have enough health to soullink with safely, and neither does the succubus; especially in AOE situations.
Hope that helps, and best of 'lock. :P
Edit: typo.
Edit2: Sample builds.