The three dots are supposed to either signify that a string of text has been cut short and that if you would like to continue reading it you need to click the dots to precede or that there are further steps afterwards as in with 'Print...'
I do see your thinking with 'Log in' but I think it's primarily used to reassure users when they think that clicking a link will do something e.g. If I see the 'Print' item in a File menu then I might be scared to click it in case it starts printing 30 x A3 copies of the page I'm on by accident, so the '...' signifies that I haven't committed to the action yet as there's more choice to come. With 'Log in' however you fully expect there to be more steps and you don't need he reassurance to continue (you know you can't log in without credentials) :)
In relation to web applications it's probably something you need to judge on a case by case basis i.e. is this an action that user might be wary of committing to just with one click and can the '...' help? But equally think about the opposite scenario such as 'Sign up...' - this would perhaps signify that there is a long process beyond the button and actually might lower click throughs and conversion.