Infraction: 6. Do not post unless you have something new and worthwhile to say.
Message to User:
Most of your post is fine. But your comment, "Generally speaking, although I haven't done any ToT, there's at most one boss where we can't get behind" should have been omitted. If you don't have personal experience with something, don't guess. You may be wrong, as in this case.
On Positional Requirements and Unpopularity:
As long as there aren't more than one or two fights where you can't use Backstab or Ambush for the majority of encounter, I don't see any issue with the positional requirements. Generally speaking, although I haven't done any ToT, there's at most one boss where we can't get behind. Consider that Sub was fine for DS even though Ultraxion was a bitch. Of course, that's because sub had the highest damage, but that is how it should be - Sub has highest damage behind the target, lowest facing.
I think sub's main issue is chronic unpopularity. There are often questions on the official forums, even back in DS, asking what the PvE spec is - Assassination or Combat? - and never considering Sub. Of course, there's other reasons for Sub's unpopularity, it definitely has some issues in that it is less versatile than the other specs, but historically most people think it is significantly worse than it is.
On FW Uptime and Saving Vanishes:
Interesting thought, we definitely won't know the uptime though for a while, but let's math some stuff out. Assume 25% haste and 100% uptime on SnD. Also, we are getting the equivalent of 5 energy every two seconds via our 2pc bonus. So, that's 12.5+4+2.5 = 19 energy/second. Let's bump that up to 20 energy/sec. Also, let's assume finishers cost 0 energy. This means we'd have to get 4 BS's off and an Ambush in the 10 seconds we have FW. That's 35*4+60 = 200 energy required over 10 seconds. If we have 20 energy/second, that actually works! Of course, the numbers came from my ass, but if the set bonus stays anything like it is now we could have an interesting time.
Of course, Vanish will still be very useful as it provides MoS, a longer FW, and Ambush might be better than BS even with the energy cost due to its higher damage and higher CP generation.