The point is that wiping dots is such a minor niche that we really don't need two spells that do it. And with Spike being neutered one mechanism at a time, more and more it'll be used only to wipe dots.
I'm going to give even the core concept of Insanity a thumbs down; even after it's functioning correctly I can think of only one place it's truly worthwhile and that's burst for PvP (we don't need any more burst in PvP, we're already scary good in that department). I can see how it'll be neat "sometimes" in a PvE rotation, such as when both dots have under 3 seconds left anyway so you can get the most out of them before they fall off. But how often will both dots really line up like that? Or will the final scaling for Insanity be such that it'll be worth the wasted GCDs of reapplying dots even if one of them has half or more of its duration?
Overall it's weird and confuses me. Like a lot of other stuff Blizz has done with Shadow, it makes me scratch my head and go, "how am I supposed to use this?" It's not intuitive, it's funky and makes you devise ways to use it. That's not good spell design IMO.