The problem with putting such a rich melee system into the game is lag queue. It would get really unfair if you were to get the slightest lag bug during a melee encounter in multiplayer, causing the timing for certain attacks/counters to be impossible. In the single player we know that the melee is in real-time, theres no quicktime or beastmode. I guess it could be compared to Batman Arkham city's mechanics. Block, uppercut, step etc. Having this kind of thing in multiplayer wouldn't work all too well. If the host has a rubbish connection then melee is a death sentence.
I don't think the mechanics work like this, at all. From what I've seen of the SP videos, it doesn't seem to be a full strike by strike and defense by defense melee system, like Batman's. It seems very different from Batman's and closer to the Multiplayer of Assassin's Creed 3. It seems to be about who starts the attack wins. And if both start relatively at the same time, both take some sort of damage and some other criteria ends up determining who the victor is. So, lag would not be a big problem.