Thanks for the clarification, Joshin. It has more to do with what I call "the front door" than it does with anything else. The answer is complicated -- many-layered, like the way the Buddha teaches! -- but definitely not academic, mostly because I'm not just concerned with how what the Buddha taught is received and practiced by secular Buddhists, but I think about everyone: all current Buddhists, and those who will be approach it with interest in the future.
Sometimes I get accused of being a Buddha-worshipper, or making arguments from authority: What difference can it possibly make what he "actually" said or meant? I have huge admiration for the man, but I don't worship him; I don't consider him an ultimate authority (I would not defer to him on a point I thought he made where he was wrong, so he's not, for me, that sort of authority).
The argument I am making is that what I learned early on of his teaching -- just the very-most basics -- has been extremely sensible, clearer and more straight-to-the-point than anyone else's system; and usable too. When I look more deeply, I keep finding it even moreso. The more deeply I look the better it gets; in particular the more effective it is in my life. I'm not saying that we should look deeply at what he says "because he's an authority" but "because he's damned good at seeing us clearly and helping us see ourselves clearly".
Point one about this is that the better we understand what such a smart man was saying, it seems likely the more helpful it will be. Point two is the one relevant to "what the five aggregates are". Point two is that if it can be shown that he really did have an entirely pragmatic, secular approach -- that rebirth really wasn't an integral part of his method -- then when those becoming interested in Buddhism start exploring what it's about (before they've even quite understood that what matters is not what some authority says is important, but how the practice works in one's life) they'll feel as comfortable exploring secular Buddhism as traditional Buddhism. Whereas, right now, what I am seeing is that many newcomers aren't drawn to secular Buddhism because it is perceived as inauthentic. They start off with the classic questions about how rebirth can work where anatta is understood, and when they get both answers (the traditional -- but authentic! -- tap dance, vs the secular -- but made up! -- logic) they go for the tap dance because it sounds so good and has a 2,500 year history that gives it legitimacy.
All this secular talk about "bracketing off" some of the Buddha's teachings and "re-inventing it from scratch" and coming up with a "Buddha-inspired system that is a better fit for our times" is all well-and-good for someone who knows the people involved, their intentions and methods, but is off-putting for those just beginning their investigations.
And where this leads: the difference between practice-with-belief-in-rebirth and practice-with-an-agnostic-approach-to-rebirth is HUGE. I know it is the PC thing for a secular Buddhist to say, "Oh, we love the traditions. We don't want to change a thing about the traditions." But I'm not *really* a secular Buddhist (I'm a skeptical Buddhist) and though the traditions may want to squash me like a bug for saying it, I do hope they'll come to see that teaching rebirth undermines what the Buddha was trying to get us to see in a really significant way.
This is why "Are the aggregates ever a parts list or are they only ever about how we think about what makes up a human?" is so important, because it is one of many key points that will show us that the Buddha did not want us to spend time living life as if rebirth was a fact -- because (1) that worldview puts the emphasis on doing good deeds for all the wrong reasons (good deeds are good! but the reasons foster a concern with self) and (2) it teaches people how to look for evidence and fit it to theory rather than look at evidence and let it knock down theory if it should.
The last thing I would say about this is that I get told to beware that I'm not inadvertently "inventing the Buddha in my own image". I understand why this is said; I understand that it could be so. But I also have seen enough of what's in there to be pretty certain that the Buddha was actually pragmatic and secular -- the reason I was able to uncover what (I am, again, pretty certain) is the original structure of dependent arising when no one else has managed it is *because* I've seen the way so many of the pieces fit together in that secular, pragmatic way. Because I don't let the traditionalists and the naysayers take over my brain and believe them when they say, "2,500 years of teachers can't be wrong..."
I keep the doubt they plant in place in my brain to keep me paying attention to how I'm researching, and what my motivations are, but I have to keep this in mind, too: If the Buddha *was* a pragmatic, secular teacher who took an agnostic approach, how will we ever see that if we all fall for the "He wasn't -- all our teachers say so! -- so don't even bother to look!" line?