I have an answer to this, and it's been a point of concern of mine since BG3 started getting gameplay videos. What's lost here is legitimate roleplay of a custom character, to some degree. Not because it's worse than other games, but because it's worse than an origin you can play as.
I certainly get what you're saying, and don't disagree in the slightest....but by the same token, in this regard a CRPG that has 'origin' or fully unique crafted characters, is always going to have this issue to some degree vs say what you can do or roleplay in a PNP/tabletop context. Larian unfortunately can't step in on the fly with everyone and play a live DM role, or create voice content for everything in a custom character context that we might think of or want to say or do in response on the fly.
So, getting back to origin vs. custom and whatever options are available to the first vs the 2nd, to clarify what I meant by 'what are you really missing', you'd first have to know every possible permutation of an origin and be aware of it to know you'd be missing that, but also, if you want those experiences, and we'll go back and use Astarion again....then you can play as Astarion and get those experiences. The only side angle of this where I could envision this angle perhaps being an issue is if, again, they were going to later come out and say (sticking to this example and the context therein) vampire spawn, but you wanted to say make and play a female VS instead, or a different looking one, with a different name, but yet wanted to experience Astarion's same 'box set of permutations' we'll call it. In that instance, that would certainly be achievable, if Larian is willing to go the length to provide that variance against whatever full roster of origin chars are in play and provide that kind of customization/'plug in' factor.
Otherwise, even the origins are limited to the same pre-programmed 'box set' of permutations available too in their own way, even if we don't know what those are yet either. To me, the real meaty question boils down to, are custom character combinations going to have their own set of pre-programmed permutations box set that is wholly and separately (to varying degrees at least) divergent from origins, or rather, can I experience things on a custom that I can't or won't on a given origin? Let's take Gale, a human male wizard....if I create say a female high elf wizard with a completely different background from Gale's, am I going to experience different possibilities and permutations within the overall box set, that Gale never would/could if I played as him instead?
If Larian's answer to that ^ (whether in an interview question or eventually discovered over time through gameplay experience down the road) is a yes, I'll be satisfied, speaking for myself. But all that remains to be seen at this early stage.
