That I agree with, shouldn't assassins wear light armor?
Actually assassins that are attempting to sneak on their victims would wear a material like wool, since it is far quieter than (lol) cloth robes and allows maximum flexibility.
However, when assassinating in broad daylight in crowded streets - the same thing as any other crowd member. With just enough room for a few concealed weapons - not an arsenal the size of Russa mind you.
It's important to remember Assassin's Creed though, wasn't really going for any kind of realism.
But I feel skyrim tone it down a bit, they don't like to go too high fantasy.