As mentioned by the previous poster, we will not always want to add the brackets. Besides which, having accepted the completion-word, we only need to type an opening bracket.. So I hope that this feature is not added
The thing is that if this feature was implemented, it should go a step beyond and also generate an "automatic snippet" with all methods parameters so you wouldn't need to go and check the method definition. That way it should be pretty easy to erase all parameters if you wanted to use the method name as a reference, a parameter in another method, etc. Of course this should be customized in languages where you don't need parentheses, but you'd get the benefit of the automatic parameters too.
What you are suggesting is the way it's implemented now: you don't get auto complete with methods from other files, you have to manually go check the method definition, and write parentheses, parameters and commas.
I really don't see how having to manually go check the definition and writing everything is better. It's like going back to notepad IMHO.