Well, the number one parse for warlocks on Mythic Dragons is by a dude using that build, so there seems to be something to it. The top 50 lock parses in general have a good number of guys using that build or something similar. I wonder if their guilds are using some strategy to pull a lot of adds together, otherwise I'm not sure how StS could be worth it, but there's gotta be some way, because these dudes are doing some pretty crazy damage.
Edit: Compare these parses:
"Meltzu", #4 Warlock parse, pretty much pure ST build
"Mysterx", #7, the mentioned AoE build
The AoE build seems to have an edge when it comes to Spirit Shades, but that has to be taken with a grain of salt since there was one Shade spawn less in the first parse since the fight was shorter. Overall I'm quite surprised by how little difference there is when you compare boss damage to add damage between these two builds that seem pretty drastically different.
Edit2: Upon closer inspection, the only difference in playstyle seems to be that the AoE build uses pure Seed spam for the Spirit Shades. And, of course, using Phantom Singularity during Shade of Taerar/Essence phases. But it doesn't seem to be a real damage improvement for anything other than Spirit Shades.