I try my hardest to make sure that if I shadowtep, I lay a rupture after it. For example, if you have to run back after a knockback, and you just shadowstep for more on target time, pop a couple hemos to get a 5 point, and use rupture.
I'll put it a different way. Putting a rupture down while NOT in shadowstep is around a 500 DPS loss on a boss fight. You can still use it for utility, just be ready to refresh a rupture half way through the duration (this syncs the Rupture and Shadowstep cooldowns back up).
If you didn't use shadowstep at all, it's a 500+ dps loss. You lose DPS on both the rupture ticks and the melee swings / poisons that happen natively.
As for viability in the long term, anything is possible. It seems that Blizzard truly has brought all of the DPS specs to within a few % of each other now. Sub excels in high mobility fights because even if you're out of range of attack, if ANYONE is attacking that mob, you're getting combo points. Even if you use the spares to refresh a slice and dice, they're usable.
Oh, on SnD. It's not as important as in other specs at all. I actually find I do more DPS without wasting the combo points on SnD, using them on Eviscerate and Rupture only. You get a LOT of combo points, but the DPS increase of adding .2 speed to your daggers doesn't compare to the eviscerate damage, especially if it crits.
A proper Shadow Dance is also very vital. When you pop shadow dance, ALWAYS do so at full energy. You want to alternate between Ambush and Eviscerate (if you're in a raid, the eviscerate IMMEDIATELY after ambush should usually be at 5 points. You can get unlucky, though, and this kills your DPS). Timed properly you should have gotten 3 ambushes in. My favorite way to start a fight as Sub (boss fight) is while in stealth, pop premed, use your tricks macro for the tank (this is going to BOMB some threat!), and as SOON as the fight starts, cloak if necessary to stay in stealth, shadowstep (usable in stealth, as is tricks), immediately ambush, this opens up a 5 cp rupture while still in the SS buff (pop trinkets first if you have them), rupture, shadowdance, a/e/a/e/a/e vanish ambush prep ss rupture, vanish ambush eviscerate, and continue your normal "keep my energy above 35" rotation from there.
I've seen DPS bursts of 8500 DPS with EXTREMELY lucky crit streaks.
Compared with other cooldowns in the game, SS and SD have extremely short cooldowns, so it feels really exciting when you get to use them multiple times in the same fight. Prep, however, is on a very long cooldown.
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"Saying that a druid can go resto and do well in arena and saying that a hunter can't do the same doesn't make any sense whatsoever." - Wait