"In summary, the arguments for copyright assignment start to look decidedly weak." Honestly, this article did not give a fair look at the benefits of copyright assignment to a non-profit like fsf, and I am sorely disappointed.
While I am not going to make a comprehensive list of copyright assignment benefits, a few things off the top of my head I think can demonstrate how out of balance this article is.
* As BKuhn said, it makes enforcement easier "linearly", which is a really big deal. The article deserves a correction, and we deserve better from our author.
* The article suggests that developers would potentially be better off enforcing copyrights themselves. But we know from BKhun that non-profits are doing the vast majority of all enforcement. This itself seems to prove otherwise overall. The article mentions a developer complaining that FSF didn't enforce something, then states: "Once a developer assigns copyright, they are at the mercy of the assignee to enforce the copyright." The strange and false implication being that the FSF's would generally actively prevent people from helping them do or fund enforcement. If this developer owned his copyrights, that wouldn't magically make someone else do the enforcement for him like he asked of the FSF.
* Dismissing relicencing as a rare and unneeded benefit, I believe is quite short sighted and wrong. Consider effectively indefinite copyright, our legislature more and more taking up laws governing technology, new technologies, and eventually people dying and becoming truly inaccessible to agree to a licence change.
* "Or later" doesn't mean that a licence shouldn't actually be changed to the later version.
* Another thing I actually haven't seen mentioned much. The idea behind copyleft (as many envision it) is that restricting our freedoms with respect to software is wrong, and in fact, it is wrong enough that we should employ our legal system to stop it from happening. However, a copyright licence cannot accomplish that task for the copyright owner. So, giving that ownership to a nonprofit which is legally bound to fulfil its mission of software freedom, is like an even stronger version of copyleft, another step to strengthening that software's freedom. For example, many free software developers have or will write proprietary code for some company, and they have free reign to use any previously copylefted code they own the copyrights to in order to do so.