In theory if you critical while doing a backstab you are more likely to one-shot your target and thus have the chance to trigger the assassination animation (though I have become quite fond of the face to face animation as opposed to the from behind one).
It has been found out that most of the perks in the one handed tree actually do not affect daggers because of the bonuses they get from the assassination perk along with the Shrouded gloves making them do 30x damage total when used from stealth. Despite this, it should be noted that a critical strike from an assassination attack has been noted to do around 32,000 damage (which is why its understandable that the perks do not affect them :))... Even on Master with all the ups and downs there is nothing in the game that could actually survive that.
I have a set of Daedric daggers that are legendary, but they were just crafted with 100 skill, maybe a 10-15% smithing item I might have found along the way but no real effort into maxing out the stats on them.. I have had no problem one-shoting anything I have been able to sneak up upon with these.
I would say that in general the Critical Strike perks are a waste of perk points for an assassin. Further more an assassin should want to keep things on as much of an even keel as possible. If you are relying on a critical strike and it only happens 50% of the time, then you are going to fail half of the time. Much better to just work on getting a nice set of daggers to lead with so you can consistently kill anything that does not see you.