Assassinations are a travesty in gaming. They are pointless and take away extremely valuable time. The time it takes to complete the animation of an assassination is enough time for the entire enemy team to destroy you with team shots. I couldn't imagine the number of times I accidentally held the melee button down a fraction of a second too long and performed an assassination knowing full well that there were enemies right around the corner. As soon as that stupid animation would start I would set my controller down and wait to die. It's unavoidable. I hate feeling helpless. That's my two cents. Don't get me wrong, they look cool... But in my world every second counts. There are far more important things to focus on IMO.
There should be a seperate button to press to do a assassination, which wastes a button essentially. I do however like them when im safe to perform it. Also if your able to be shot by the entire enemy team then you wouldnt make it very far anyways.