yes the avengers membership should be open to everyone but then when new avengers keep on being added like star brand, new iron heart and coming and going then the tenure doesn't leave much of a impression once they leave so to have a character make a greater impression they are adding to avengers classic teams retroactively. There is another danger inherent in the approach that anyone will become an avenger irrespective of experience or long term investment as sometimes it may lead to disasters like the sentry ad avenger X who later turn out to be their adversaries.