Compared to combat there are several good reasons to play assassination. Better damage when you can't stick to your target (Thok p2), not having combat weapons, being in a raiding group where others need the combat weapons, not having AoC, execute phase, and most importantly: your fingers.
Did you remember to set "time in execute range" to 1? Assassination is really strong in execution phase, with or without cooldowns. I'd recommend assassination for progressing Garrosh normal unless your combat weapons are better. It's just so good in the part of the fight that matters.
I looked at that too, but no I wasn't counting that because Verain (I checked the spelling to make sure I got it this time just for you <3) was already accounting for the gain specific to having dispatch for the entire duration. Thats where he got his 1.1x normal mut damage
Unfortunately, its not really possible to go into it further because comparing across specs isn't perfectly accurate and the results will depend on the gear used and his is unknown. Though, I was using my current gear and switching for equal ilvl daggers. He said his combat ilvl is higher which quite likely means higher ilvl weapon as I don't see any other slot thats higher ilvl and good for combat losing to a lower ilvl piece for assassination (though obviously the reverse is not true).