Depends on your party make up.
Useful for one line each over two mages (support gets hex, nuker gets death cloud). You should stack spirit damage equips to max (ie. 30) anyways, since spirit is the least resisted and AFAIK noone is immune to it, so it should be your main nuke line. Arcane Bolt, Drain Life, Mortality Curse, Crushing Prison, Mana Clash, and Death Cloud are all spirit, giving spirit the most amount of spells to increase damage with equips.
Apart from that, the small increase in burst isn't worth having multiple iterations of the Death Cloud and Hex lines.
I dunno, someone posted 1k damage with it, so if you can do THAT much, it might be worth it. Through 'testing' (ie. cheating with an 'offline account' and give it a test run), I've run around same numbers as Odd Hermit even with max of spirit equips and . Not nearly 1k, but I've heard some monsters are more susceptible to it. :shrug:
I mean, going by code alone, the combo's base damage is 200 + 2 per spellpower, if I'm reading code right. A max magic mage with endgame equips should run about ~110 spellpower, 115 with spell wisp, and 127 with spell might. That should be around 454 base damage, 590 with equips.
Affliction + Vulnerability hex is -0.3 and -0.2 resists, but I'm not sure how that works, as I've yet to find the script for spell damage and how affliction/vulnerability works. Assuming that -0.5 is -50% resists (1.5x damage), and that the enemy starts at 0 spirit resist, then thats 885 damage... still short of the 1k.
Of course, if you run a 1 mage party, having 3 wasted spell points on the guy that you're falling back on for healing/CC/AoE isn't very ideal. I say just drop the combo altogether in this case.