Library uses a JPEG preview for speed and offline viewing. You can increase the quality of that preview by going for a larger size and higher quality, reducing the difference between Library and Develop (size, compression, and there's also a colour space difference as ProPhotoRGB doesn't squish into an 8-bit JPEG preview well), but then you have a speed and space trade-off because those previews are then bigger files, so you have people complaining that Library's then slower.
Develop always uses raw data for quality, and I agree the Develop preview could do with some attention to give the accurate view it deserves.
If, as it seems you're suggesting Rob, the cached 1:1 raw file (no JPEGs, same issues otherwise) was added to the catalog's preview cache for every image, the preview folder would be HUGE. Now if you wanted to take some photos out with you for offline viewing, you'd have to take these huge previews, in which case you might as well take the raw files and not cache the raw data at all, no? And why spend time and processing power reading and processing raw data to create a Develop quality preview when you only want to add a keyword in Library module?
There's going to be a trade-off somewhere - the debate is whether it's in the right place. If the modular concept was dropped, and Library and Develop were merged, then the whole preview system would need merging, but if Adobe are sticking with the modular concept, which seems likely, then leveraging the benefits of faster previews in Library and accurate previews in Develop seem to me to be the most logical.
So the question is, what's the crux of the matter here? Is it that Library previews aren't fit for purpose (rapid and offline viewing) or is it that the Develop preview needs to be more accurate at smaller zoom ratios, so that users don't have to flick back to Library to see the zoom they want? The OP seems to be requesting the latter, and personally I think he's got that targeted correctly.
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