I don't think this would really be worthwhile to implement, especially since it's not adding new information that's not already accessible in the profile.
We got the field back, in a granularity that presumably makes privacy-conscious folks (more) comfortable.
The down-to-the-second value is still available via the API for anyone so inclined, making it accessible to power users and userscripts (eg. LegacyProfiles) for those who miss the feature.
The down-to-the-day granularity is also still fully accessible on the account list on public network profiles:
As mentioned by @TylerH, @AaronShekey (the lead designer for Stacks) implied in a comment that development cost was a significant factor in choosing the current implementation over others, and offered that what we now have was the cheapest solution they came up with while still keeping the feature around at all.
This request would add further development cost to a feature that, at the end of the day, seems to address the current, popular use cases just fine. I don't think we need this.