Writing idiomatically generally makes your code more readable to people who are more experienced in the language. Your version, while not incorrect, will make most people do a double take. Kind of like messing up an expression, like saying "Make a tree and go!" instead of "Make like a tree and leave!" People will still know what you mean but the way you said it is not quite what people are used to.
EDIT: I guess "must" really is "should" -- you won't get any compile-time errors if it isn't final but making it so can protect you from introducing problems later if the code is modified and extended.