The motivation for multi-volume was actually just various flavors of MRI in the context of prostate imaging. DCE timing interval is fixed, but other applications for prostate imaging include various T1- and T2-mapping sequences (variable FA or TR), and DW trace images with unequally spaced b-values (e.g., see Evaluation of fitting models for prostate tissue characterization using extended-range b-factor diffusion-weighted imaging - PubMed). In multivolumes, from the start, the forth dimension was not intended to always correspond to time.
I agree with many points, but I think once you want to step out of the boundaries defined by those formats, and introduce extra complexity, in particular - handling metadata (like the issue we are discussing here), the answer is not trivial. I am not aware of examples that use NRRD or NIfTI (or BIDS, for that matter) for large-scale data aggregation and metadata management.
The question becomes do we want to invest the time to further extend those formats, or explore how those situations can be handled in DICOM. Yes, vendors have different implementations and interpretations of DICOM, but why not try to establish best practices and open source DICOM conversion tools that we can iteratively refine? We have just a handful of established tools that normalize vendor-specific DICOM and output NRRD/NIfTI - why not explore how those tools can output multiframe DICOM in addition to the research format they output right now?
I agree at this point it is hard to make the case for using DICOM to solve this specific problem, since there are no converters that could “just do it”. But I would much rather explore how this problem can be solved using DICOM. And if we have more people who believe this approach is worth the time investment, we would much sooner be able to decide how suitable multiframe DICOM is for those tasks, how much effort it will take to implement it, and how to proceed with refining the standard along the lines you mentioned, Chris.