I believe this has to do with the HUD. When a player is on a killing spree, it'll flash up in the middle of your screen "Killing spree" and on to "rampage" and so on. Now this is wonderful and all. It's a typical fighting arena staple. And i like how league of legends takes it seriously and doesn't *******ize it like Heroes of Newerth does.
Why must kills get all the glory? If i get 7 assists in a row without dying i want a little something. Perhaps a "VERY HELPFUL" can flash across the screen.