First and foremost is the problem that the late formulation of the 13 Ikkarim in no way negates the fact that the halchic status of a heretic had already existed for a considerable length of time. Accordingly his inference that we should not be/feel compelled to affirm and particular set of beliefs from the late dating of the 13 Ikkarim is a total non-sequitur.
His statement, "I promise you that almost all the observant people you know would have difficulty if they were asked to verbalize their precise true beliefs and have them compared to the "icarim"." clearly falls into the category of an ad populum fallacy. The success or failure of people to accept something does not negate it's truth, nor does people's lack of faith negate the obligation to have faith.
The truth is that, "rationally" speaking the question isn't whether one should affirm ikkarim when one finds them, well, less than compelling. The much more significant question is why on earth would someone affirm a world view which asserts numerous points which they disagree with even if they are inclined to agree with a skeleton of "fundamental" views? The question isn't whether the ikkarim are sufficient or necessary for being Jewish, it is whether it makes any sense to affirm Judaism when one disagrees with it on any issue up to [and maybe including] the ikkarim?
It is especially perplexing when there is an alternative religion, Conservative Judaism, where such a theology is normative and one's observance is considered acceptable. Don't get wrong, I guess I'm kind of a big tent guy, but that is a matter of wanting to see people closer to the truth rather than farther. From a logical standpoint I just fail to see the appeal for people to affirm a belief system when it requires so many astrixes.
I believe [the] comment raises some significant questions on how to proceed in actualizing a modern day "Rationalist" Movement envisioned. The fact is that many of those most excited by the prospect, and active in the dialectic, are those who have a tenuous relationship to fundamental principles of Yiddishkeit. I'm not talking about those who question the Rambam's enumeration, but those who have effectively dismissed the notion of heresy. This was an issue for the Rambam's version of Rationalism as well, with people who considered themselves his successors denying that which he affirmed (such as creation Yesh M'Ayin), and attributing such beliefs to him.
Such people help with momentum but compromise the theological integrity of a neo-Rationalist movement. And while it may not be exactly a logically inevitable, it is realistically inevitable that the halachic integrity will be compromised. While one might argued that committed orthodox rationalists will conform to kabbalisticly influenced halachos in anticipation of the day when a kosher Sanhedrin returns things to their "proper" order, this strikes me as highly unlikely and unattainable once we introduce a significant number of members who do not really anticipate the coming of Moshiach or are otherwise not ideologically committed. (I do not mean to limit halachic compromise to kabbalisticly influenced psak, but it would be the first.)
I guess what I'm saying is, be careful what you wish for Rationalists.