> But in the Bibtex file it is very common for the titles to appear in their original title case form
That is common because they are following the rules about how to steer capitalisation when using bib(la)tex:
- If the entry is in English, and the style demands title case, output as is
- If the entry is in English, and the style demands sentence case, convert to sentence-case and output
- If the entry is not in English, output as is
> If the entry is in English, and the style demands sentence case, convert to sentence-case and output
Nope: not possible to automatically determine which capitalised nouns are proper (and thus remain capitalised in sentence case) and which are common (and thus become uncapitalised).
This is in fact why it is better to store sentence case: it can be unambiguously converted to title case while the reverse is ambiguous. It’s not mere preference.