Ghostcrawler's quote from the Mind Sear thread on the official forums.
We want Shadow to use multi-dotting in addition to Mind Sear when there are only a few targets. When Mind Sear is too strong, then the "rotation" becomes Mind Sear -> Mind Sear -> Mind Sear, which isn't a ton of fun. If you're AE'ing lots of targets, then it's fine to do nothing but Mind Sear (and perhaps a level 90 talent).
This won't work. This is basically the same idea that we had going into t11, and what ended up happening is that those specs who's AOE was designated (arbitrarily, imo) as "fun" were useful, and everyone else just had terrible AOE (like Shadow Priests, Retribution Paladins, etc). There were several fights where Shadow just wasn't able to do the job of AOEing in t11 thanks to a weak Mind Sear (Cho'gall, Maloriak, Magmaw, etc). And that was when our DOTs were actually OP. Now, with weak, expensive DOTs and weak Mind Sear combined with weak mana pools...yeah AOE is not going to be Shadow's strong suit in t14.