Well, they could do that before actually. There was a passive that only assassination had for backstab (the energy cost), and a similar one for subt. Both were out of reach of the other and mandatory in pve. So they always had those knobs.
The dispatch addition did the following:
1)- Removed positioning. This is HUGE, because we finally have one spec without this concern.
2)- Got us our energy bar back. With 120 energy and dispatch, we have well over half the bar to play with. We can pool, etc. With the only energy ranges we cared about between 60 and 120, we had a really odd situation.
3)- Bought us a move in pvp. The previous combinations meant that you would often not even spec this execute in pvp- not only was it hard to use because of positioning and health levels, but you would normally go from 70 to 100 energy before your first one would even land- and any CC would then destroy energy as well. You were often better continuing to mutilate- at least that would get you combo points for sure!
4)- Bought us the proc, though this could have been done with backstab.
Well, an eviscerate nerf still hurts BOTH C and S.can nerf eviscerate without touching assassination etc