Actually, it's not about typical caller behavior. When you write a function, what 'this' is is going to depend on how your function is called. If you don't want caller behavior to affect your function, don't use 'this'. If you want to allow callers to choose what object your function should act on using a variety of convenient syntaxes, then write your function to use 'this', but be aware of all the ways a caller might set it.
