if there is a company doing major development and you don't do copyright assignment and they go under and you want to try your had at proprietary licensing, you have to buy all the copyrights that you don't own. It may be harder to buy copyrights from many different people than to buy them all from one entity (even if it includes things that you wrote to begin with)
there is a fundamental difference in view between the people who assume that assignment (even if joint) is valid and those who do not
those who see it as valid see the organization that manages the copyrights as creating the vast majority of the code, with the other contributers being , if not minor, at least significantly less significant.
those who see it as invalid see the contribution from the outside as being worth at least as much as that generated by the organization.
different projects will have different ratios, but at least initially, almost every project where their is a team of people paid to work on it full time, that team will out-produce the outsiders. Over time the outsiders may become a much larger portion of the development, but if the development really is very lopsided, is it really so unfair?
one thing that assignment avoids is any arguments over if a particular patch is significant enough to warrent copyright on it's own.